I compared this to my current go-to Bluetooth headset, which is the Plantronics Voyager PRO (PV Pro) Bluetooth Headset.  I've made some direct comparisons between the two.

Overall, I found this headset to be "very good" to "excellent" with some caveats.  I would give it a strong 4.5 stars, but because of some of the limitations, I cannot give it a 5-star rating.

I highly recommend it.

Pros
-The voice guidance was a great help during setup and pairing.  (Not available on the PV PRO)
-Compact size and design.  Significantly smaller than many other Bluetooth headsets I own.  (PV Pro seems like a huge contraption compared to this)
-Pressing the middle button once tells if the headset is connected.  Keeping it pressed for a while will activate voice commands on the phone (voice dialing, check battery level, etc.).
-Very good/excellent sound quality; I didn't fine the sound too muffled, but that is personal preference.  I found the PV Pro is a touch better in voice quality and clarity.
-noise cancellation is very good when driving or in a noisy office environment, though not the best I've heard (PV Pro wins this)
-the switch to turn the headset on/off is a slider, which reduces the chances of accidentally turning on the headset (unlike my Jawbone Prime, which turns on if kept in my pocket)
-audible prompt that tells me "Power On" when I turn the headset on, "Connected" when the Bluetooth connection is established, and "Power Off" when I turn off.  When I press the button on the opposite side of the power switch, it tells me "Batter level is High."
-Communication between phone and headset; Jabra Extreme2 tells me who is calling, if the caller is on my contacts list.  It also tells me "Call ended"
-Talk time and standby time is amazing!  I've gotten over 5 hours of talk time on a single charge (rated at 5.5 hours).  I've left it alone for a week and a half, and powered it up with "Battery level is High" prompt.  However, the PV Pro seems to have a longer talk-time, but at a cost of having a larger battery and making the unit bulkier.
-Transmits music and audio from my blackberry and iPhone.
-Battery indicator is visible on iPhone 4S (but not my Blackberry)

Cons
-UPDATE 2011-12-29 -Activates Siri on iPhone 4S, but Siri cannot "hear" me.  Voice activation with EXTREME2 on Blackberry works perfectly fine.  PV Pro activates and Siri, and recognizes voice prompts.
-The noise cancellation sometimes chops up my voice on the other end if it is windy outside.  This feature is supposed to be the unit's primary selling feature.  It is supposed to have HD voice technology, which is supposed to give the caller's voice with "unbelievable clarity" which I found to be just ok.  The voice was clear, but it also sounded somewhat artificial.  This may be why some described the voice to sound "muffled."  According to the manufacturer:
"Extreme Noise Cancellation for Any Environment:  Jabra's Noise Blackout 3.0 technology blocks out an astonishing amount of background noise, to ensure you can enjoy your conversations without disruptions. To create the clearest call possible, the headset's dual microphones capture sound while intelligently filtering out background noise. This eliminates ambient noise even in the loudest environments, enhancing the call clarity and ensuring a natural sounding voice."
-The earloop does not always give a proper fit in my ear.  The earbud occasionally comes out of my earlobe and the headset just hangs from my ear.  It just needs the proper angle and configuration, and once it is there try not to move the head too suddenly.  I do not have this problem with the Plantronics Voyager Pro.
-The audio is not as loud as the Plantronics Voyager Pro or the Jawbone Prime.  Again, not a major limitation but it is there.
-For some reason, when I watch a YouTube video while paired with this headset, the video is sometimes blacked out.  This does not happen with any of my Bluetooth headsets.  It may be a safety feature, where if paired with the phone it is supposed to keep the user from watching a video while driving, but this is just a speculation.
-Does not give a battery indicator to my Blackberry (but my PV Pro does).  However, the EXTREME2 gives a battery indicator on my iPhone 4S.  Many of the newer Bluetooth headsets give a visual battery indicator on the phone (Plantronics Voyager Pro+, Jawbone ERA, Jawbone ICON HD, etc.)

Disclaimer: I received this product through the Vine program in exchange for an objective review. I do not receive monetary compensation from Amazon or its affiliates. I spent my own free and valuable time testing and trying out this product and providing this objective review.
I compared this to my current go-to Bluetooth headset, which is the Plantronics Voyager PRO (PV Pro) Bluetooth Headset. I've made some direct comparisons between the two. Overall, I found this headset to be "very good" to "excellent" with some caveats. I would give it a strong 4. 5 stars, but because of some of the limitations, I cannot give it a 5-star rating. I highly recommend it. Pros
-The voice guidance was a great help during setup and pairing. (Not available on the PV PRO)
-Compact size and design. Significantly smaller than many other Bluetooth headsets I own. (PV Pro seems like a huge contraption compared to this)
-Pressing the middle button once tells if the headset is connected. Keeping it pressed for a while will activate voice commands on the phone (voice dialing, check battery level, etc. ). -Very good/excellent sound quality; I didn't fine the sound too muffled, but that is personal preference. I found the PV Pro is a touch better in voice quality and clarity. -noise cancellation is very good when driving or in a noisy office environment, though not the best I've heard (PV Pro wins this)
-the switch to turn the headset on/off is a slider, which reduces the chances of accidentally turning on the headset (unlike my Jawbone Prime, which turns on if kept in my pocket)
-audible prompt that tells me "Power On" when I turn the headset on, "Connected" when the Bluetooth connection is established, and "Power Off" when I turn off. When I press the button on the opposite side of the power switch, it tells me "Batter level is High. "
-Communication between phone and headset; Jabra Extreme2 tells me who is calling, if the caller is on my contacts list. It also tells me "Call ended"
-Talk time and standby time is amazing! I've gotten over 5 hours of talk time on a single charge (rated at 5. 5 hours). I've left it alone for a week and a half, and powered it up with "Battery level is High" prompt. However, the PV Pro seems to have a longer talk-time, but at a cost of having a larger battery and making the unit bulkier. -Transmits music and audio from my blackberry and iPhone. -Battery indicator is visible on iPhone 4S (but not my Blackberry)

Cons
-UPDATE 2011-12-29 -Activates Siri on iPhone 4S, but Siri cannot "hear" me. Voice activation with EXTREME2 on Blackberry works perfectly fine. PV Pro activates and Siri, and recognizes voice prompts. -The noise cancellation sometimes chops up my voice on the other end if it is windy outside. This feature is supposed to be the unit's primary selling feature. It is supposed to have HD voice technology, which is supposed to give the caller's voice with "unbelievable clarity" which I found to be just ok. The voice was clear, but it also sounded somewhat artificial. This may be why some described the voice to sound "muffled. "  According to the manufacturer:
"Extreme Noise Cancellation for Any Environment:  Jabra's Noise Blackout 3. 0 technology blocks out an astonishing amount of background noise, to ensure you can enjoy your conversations without disruptions. To create the clearest call possible, the headset's dual microphones capture sound while intelligently filtering out background noise. This eliminates ambient noise even in the loudest environments, enhancing the call clarity and ensuring a natural sounding voice. "
-The earloop does not always give a proper fit in my ear. The earbud occasionally comes out of my earlobe and the headset just hangs from my ear. It just needs the proper angle and configuration, and once it is there try not to move the head too suddenly. I do not have this problem with the Plantronics Voyager Pro. -The audio is not as loud as the Plantronics Voyager Pro or the Jawbone Prime. Again, not a major limitation but it is there. -For some reason, when I watch a YouTube video while paired with this headset, the video is sometimes blacked out. This does not happen with any of my Bluetooth headsets. It may be a safety feature, where if paired with the phone it is supposed to keep the user from watching a video while driving, but this is just a speculation. -Does not give a battery indicator to my Blackberry (but my PV Pro does). However, the EXTREME2 gives a battery indicator on my iPhone 4S. Many of the newer Bluetooth headsets give a visual battery indicator on the phone (Plantronics Voyager Pro+, Jawbone ERA, Jawbone ICON HD, etc. )

Disclaimer: I received this product through the Vine program in exchange for an objective review. I do not receive monetary compensation from Amazon or its affiliates. I spent my own free and valuable time testing and trying out this product and providing this objective review.