About a year ago, I dropped my iPhone 4 and shattered the screen. I replaced it with a 4S, but the 4 has been sitting around, and I decided to fix it so my wife could have a new phone.
Swimmingly. Had no issues whatsoever, the install was easy as pie and didn't take long at all!
The only issue I thought I had was a no service message. I thought I had missed a connection or something, so I rechecked all my work. Finally I decided to pop in my (working) SIM to try, and it worked. If you have any "No Service" issues, TRY A WORKING SIM before you do anything more drastic :)