Have been using a U3415W for over a week now, and I can confirm what most people are saying here. Sat in front of it, you don't really notice the curve.
However, that doesn't mean I dont think it is worth it. While I have not used a flat 34" 21:9, in that format there is likely going to be some colour shift at the extreme left/right of the monitors at a standard viewing distance. Even on the best monitors you notice a slight change in huge/brightness as you change your viewing angle. I noticed this when going from a 17" IPS laptop to a 27" IPS display. Now going from a 27" IPS to 34" curved IPS, I did not notice any of this, and in my (entirely subjective) opinion, I'd pass this off as being the curve helping to keep that shift down to a minimum.
All in all, I like the curve. It's subtle enough that you dont feel like you are looking through a lens or fishbowl (anyone remember moving from an old curved CRT to LCD?), and doesn't induce any sort of distortion into day to day work.