How safe is it to upgrade my galaxy s2 phone to Ice Cream Sandwich? So I figured that since the s2 now has ice cream sandwich available, I should look into upgrading.

Doing some research on the internet, I see that it is actually impossible (or at least quite difficult) to go back to 2.3 if I have problems. I've also seen that there have been reports of Ice Cream Sandwich making their s2 have problems such as randomly crashing or the battery runs out really quickly etc.  

So is upgrading to Ice Cream Sandwich like a fairly routine thing that ppl do and that I should just do it? Or is it actually quite a risky thing and I could seriously stuff up my phone if I did something wrong? 

Also, if it is risky, is ICS worth the risk? Is there like a massive difference or is it just really just a few UI changes here and there?
If it is the official ROMs, then it is very easy and chances of anything going wrong are slim-to-none. You should just do it. 

Downgrading should also be pretty straightforward if you want to stick with official firmwares - it would be a case of downloading PC Odin and the firmware you want from a site like sammobile. I have done this with a Galaxy Note which is from the same family of handsets.

Many people who have problems find that they can solve them with a wipe and just re-installing what they want. This has resolved battery issues for some.

ICS is worth the risk. Yes, there are UI changes and you will probably notice the phone being a bit faster to respond, but there are many other changes as well. For instance, some real-world useful features include Wi-Fi Direct, improved web browsing and proxy support. If you plan on keeping the handset long enough you may see some applications will require ICS as a minimum version as this is the first unified phone + tablet version of Android.