My 3GS was slow, locking up, resetting and lasting less than an hour on battery.
Piece of cake and now my phone is good as new!
The display does take a lot of force to lift and mine popped up suddenly, pulling some cables loose. Not a huge problem, but not idea. It pays to be really careful lifting the display. The clip on cable 3 from step 6 would not lift for me and I even broke a tiny piece off. It seemed very fragile so I didn't force it. But I was able to work the cable back into place eventually.