Broke my 4s front glass, replaced in about an hour...
The failure was pretty straightforward. I dropped a 3" PVC WYE fitting on top of it while renovating my house. Dang those things are slick. Everything still worked, but the front glass was shattered.
The repair went pretty smoothly. The battery was difficult to remove, though, and the plastic service tools from iFixit aren't really that strong. I was amazed at the minute differences between the internal screws. 1.5mm, 1.6mm, 1.7mm... Who would've guessed they'd be fussing about a tenth of a millimeter?
The old display was a bit tough to remove, the guide said to use the plastic tool, but there's really no way to insert the tool initially, so you have to carefully find a place to press the old display out, which can be difficult when it's a pile of broken glass.
Speaking of broken glass... make sure you've got a good pair of tweezers, because shards of glass end up stuck to the adhesives inside the chassis, and are a PITA to get out.
The iFixit guide is worth its weight in gold.
1) Don't break the phone :)
2) If you doubt you're ability to fix it, don't even try, it borders on being watch repair
3) Seriously, if you don't have a lot of experience w/ modern electronics, don't try it. PC repair doesn't count at all. If you screw it up, it will cost a lot more! You're not saving a whole lot of money to begin with here...
4) The pentalobe screwdriver inside the 'liberation' kit is pretty good, and the philips driver fits all internal screws, no need to spend money on the more formal tools
5) Avoid coffee before hand
6) Do it in the lab, it's easier than at the dining room table
5) Don't break the damned phone!