I used BSE on my HTC Touch Pro and was very happy. The back pieces eventually started to wear and I removed them after about 6 months, but the screen stayed good for almost a year. Some dirt started building under a corner and it started slowly lifting at that edge, but I only removed it (very slow, steady peeling to remove) when I knew I was about to retire the phone (somewhere around June 19th
I've ordered BSE (screen only) for my new 3GS and agree with steps given in the guide here. Minimal water is best (obviously) and steam is very important in the process.
I didn't order the back for my iPhone because I usually use a protective case and/or keep the phone in a belt clip, plus it's a lot harder to apply the back. But it is nice to have full coverage, so if you're up to the task, get the back also - especially if you don't use any other case or cover.
My HTC still looks brand new after living in BSE for most of it's life! BSE is a great insurance against so many probably mishaps and normal wear and tear.