My friend gave me his 4 when he upgraded to 4S. It was great, most of the time, but like a bad knee, sometimes when the weather was just right the home button would become all but unusable. Since it was a free phone and the timing wasn't right to buy a new one I decided to take a stab at replacing the button. Plus I wanted an excuse to buy a lot of sexy new tools from iFixit that I'd been eyeing. :)
Fairly smoothly. It took probably two hours. One of the hardest parts was prying the screen away from the metal bezel. It took a TON of prying and wedging little things in the gaps. Probably 10-15 minutes. I almost wore through one of the iPod opening tools. Once I felt some of the adhesive give way on one of the corners I felt confident in prying up the rest without damaging anything. I had trouble plugging in the new home button. That little connector is really tricky, and I was part way through reassembling when I realized I'd inserted the ribbon cable beneath the connector instead of inside it. Also, when putting the screen back on I did exactly what the guide warned against: folding one of the ribbon cables, so when I started trying to plug them back into the logic board one wasn't long enough. Glad I didn't damage anything.
One of the more troubling parts is some little rubber bit fell out of the phone during disassembly. For the life of me, I have no idea where it came from. About 5mm long with a C-shaped cross-section. Some sort of riser or insulator. I really have no idea where it came from, and I haven't noticed any side effects without it, but that is NOT a good feeling!
If you can get some surgical gloves they might make it easier to keep components oil free. Read through the guide first before starting so you get an idea of what you'll be up against. I was an idiot and just dove right in. And be prepared to deal with a lot of adhesive.