Should I buy a trackpad?
I have both the Magic Trackpad and the Magic Mouse, and IMO the trackpad works really well for browsing, navigating in the general system, and is a very natural way to interact with the computer.

That being said, it is not precise enough to be used when you need to do 
photo/video editing, and in those rare occasions I use the mouse.

I also do programming, but barely use the mouse/trackpad when I'm coding.

Pros: 


Very easy to navigate in pages (Web browsing, preview, etc.)
Enable one to use the built-in gestures, and that make a huge difference in day-to-day computer usage
Ideal for an "HTPC" usage of your Mac : I frequently casually use my Mac from the couch, and not having to move a mouse is a definitive plus.


Cons:


Precision
Doesn't fully replace a mouse
You'll miss the gestures as soon as you'll use a different computer.