I use a U2515H for games and compared to other IPS monitors I've played on, it's as good as any of them (the only exception being the Acer 144hz IPS).
Colours are brilliant, very deep and inky as you'd expect from an IPS. The monitor comes factory calibrated which should be more than sufficient for the majority of people. The only setting I changed was to reduce the brightness to 25%. The contrast and black levels are very good, with the only negative being the IPS glow seen when viewing the monitor in the dark, but that's pretty much the norm for IPS panels these days and is nothing unusual. Once brightness is 25% it's not very noticeable anyway, except when viewed from extreme angles. I frequently play games with dark scenes at night with the lights off, and black detail is still very good.
The pixel pitch is great for gaming and helps with making jaggies less apparent. For example, 2xAA looks just as good as 4xAA on my previous 1080p monitor. In desktop, things do look a bit small, but they're still perfectly legible. You can increase the scaling in windows to compensate, but I don't bother and just gotten used to it. In fact, when I use other monitors now, everything looks too big on them. I still think 27" is the sweet spot for 1440p, but I don't think an extra £120 (or whatever it is) for the U2715H is worth it, when they are practically the same monitor but with a 2" screen size difference.
I think if you're coming from a fast TN panel, you might find the lower response times a bit weird, but that would be the same for any other IPS panel, except for a 144hz IPS panel, but how many of those are there right now? More importantly, how many have the same quality as Dell? Personally, I find it perfectly acceptable for gaming. Response time and motion blur is very good for a 60hz IPS panel and so is the low input lag.
I play all types of games, with no major issues, including BF4 which someone mentioned earlier. Untill 144hz IPS type panels become more popular, made by manufacturers with better quality control, you're always going to have to compromise in someway. If you value image quality and not massively concerned about having the absolute best response times and refresh rates, the U2515H is a good choice and great value in my opinion.