Much smoother than I expected. Buy the repair kit!
My iPhone 5 is no longer under warranty, but Apple charges $129 to fix the display.
Almost seamless - I spent more time figuring out that a lot of steps and warnings didn't apply to me.
Buy the repair kit. I got one to do my battery and kept it. The suction cup doesn't seem to work as well as it did originally. I think it gets deformed over time. It took a few tries to get suction on the screen.
Also - for removing "sticky" contacts or ribbons from the body, the plastic pry bar isn't of much help. What works great, though, is the pointy end of a plastic dental flosser. You can even trim the point off a little to make it more rigid. It's just small and thin enough to slide around without damaging the ribbons. Then be sure to floss when you're done.