My wife dropped her 3gs and the screen was pretty cheap. I already had a tool kit from a battery replacement, so this seemed easy enough.
Getting the phone apart was easy having done that before with the battery. My kid unscrewed it. Peeling off the broken glass from the plastic part that needed to be saved was tricky. I didn't have a heat gun, so I held the whole thing in front of a small space heater until the glue got soft - this took longer than I thought. So it was heat, then scrape, heat, then pry, etc. Ended up peeling away some black rubbery trim that I shouldn't have, but overall was able to get it cleaned up. Placing the double sided tape took a steady hand, though it was forgiving if you messed up. Just peeled off and started over. After getting the glass in place, I realized it rests slightly higher than the original - meaning now the home button is recessed deeper, and not flush with the glass. My wife doesn't seem to mind this though, so I'm calling the repair a success.
Amendment: Her phone touch screen is slowly becoming non-responsive. Bummer, but despite my best efforts to replace just the screen, it actually didn't work. This was harder than I thought. Already ordered the full screen assembly to replace for about $40 shipped, which I guess is what I should have done in the first place!
Use a heat gun...would have been way better than my space heater method. If you just buy the digitizer + screen combo for $30, you probably won't have the home button issue I had.
Amendment: unless you are a semi pro repair person, go with the whole screen assembly...waaaaaay easier and definitely worth the extra $.