The hardest parts are separating the glass from the plastics and then re-attaching it. It would do you well to buy the 'custom' double sided sticky tape they sell here at ifixit. I used regular double sided tape and it took a long time to cut and fit properly so all sensors would remain working and the screen would be held in place.
I had no issues taking the 3GS apart;however, putting the new screen assembly back together was a little more difficult. I can't get the new screen to 'stay flush' with the bezel. Also the 'gasket'(that runs around the length of the new assembly) keeps getting pinched and doesn't 'snap' under the bezel properly. I'm going through other procedures to see how to remedy this.