I've just bought Dell U2713H and I regret it, deeply.
I'm using rMBP (mid 2013), connected via DP in 1.2 mode.
The first thing I've noticed is fuzziness of the image. So it happens that the monitor ships with "Sharpness" setting set to 50, which runs some vanila sharpening algorithm (on digital input picture!!!). After fixing that, I've noticed weird looking colours. There's bunch of useless modes like "cinema" or "game" (which didn't reduce input lag too much), and then there are "Standard" and "Color space" (or something along the lines).
I was expecting a well-calibrated colours with the last one, but then there are separate gamma settings (one of MAC or PC, wut?), brightness/contrast, and then there are separate colour settings (including a "calibrated" semi-option).
I know I can ditch the factory calibration, buy a colorimeter and do everything myself, but I was expecting at least "decent" colour matching after plugging the monitor, esp. that MacOSx detects the monitor and downloads color profile for the monitor, not mentioning the monitor is factory-calibrated (you get this calibration report in the box).
So after much time wasted trying to make it "look good" at least, I was struggling between "Standard" mode which gave me saturated colours and decent grays (but ugly, oversaturated reds, almost going into pink, and oversaturated blues, which had cyan tint) , and "Color profile" which gave me "calibrated" but washed out colours, quirky gamma (grays cut off) and fuzzy dark-gray (my IDE software uses a dark-gray as a background, and the text was hard to read in this setting).
But that wasn't my biggest problem... I had a full working day with the monitor, and during the day I noticed weird artifacting when I moved browser windows or black outlines on white background. You can clearly see the problem using something like Adobe Illustrator or Corel Draw, when moving a wireframe rectangle or just a thin window across a white background --- there was butt-ugly and very visible green ghosting/trailing. Even slow movement of a dark line over light background produced very visible greenish trail spanning 10-50 pixels!!!
What's more, windows in MacOSx have those small round bubble in top-left corner, where is where I first noticed white ghosting on those. So basically moving a "Messages" window (a combination of frame, text, user avatars and buttons) produces a combination of green-white-gray ghosting across the whole thing!
I've spent another 20 minutes trying to fight it, changing dp modes, resolutions, different "preset" settings, color profiles, brightness, contrast, gamma.... NOTHING helps!
The revision on this thing is A07.
I'll be returning the monitor ASAP as I highly doubt it can get any better with replacement items... unless you have any other ideas how to proceed.