My home button started being unresponsive about a year after I had my iPhone 4. The usual stuff - single clicks registering as double clicks or not at all, lagginess, etc.
Very smoothly. The iFixit guides are some of the best I've ever used. The only difficulty I had was putting the home button ribbon cable back in its socket (it's very delicate and the actual connector portion is prone to bending out of shape) and re-seating the logic board, but eventually I got everything back the way it was.
Definitely take your time with this. Keep track of every screw (I categorized them by step number) and be very careful with all ribbon cables and connectors. Wearing gloves helps a lot; there's no oil from your fingers and no need to Windex the connectors afterward. Be prepared to spend 1-2 hours total on this...it pays to be careful and there's a lot of things that could break. Changing the home button involves removing and disassembling almost every single part of the phone. Pay attention to the guide and don't gloss over any details! And also make sure you have all the necessary tools (although a spudger isn't absolutely necessary - I used a dental pick and it worked very well).