The screen on my father in laws iPhone 3GS screen cracked after a drop, so I fixed it for him.
It was the second time I had replaced a 3GS screen before so I knew what to expect. No issues came up whatsoever and the whole repair was completed in about 25 minutes start to finish.
As said many times before, make sure when you place the digitizer on the plastic frame, that the ribbon cable is were it should be! And just take your time. It's not as difficult as you may think.