As far as I know Dell Ultrasharp displays use the same good quality (or did at one time, anyway) LG-made IPS panels as Apple does.
But that doesn't mean they perform exactly the same. Other hardware differences (like supporting VGA) can negatively affect the performance. Check out C.Net's review of the two 30" displays from a couple of years ago.
Saying the Apple ones are overpriced is flat out wrong. The extra $300 between the 24" ACD and 24" Ultrasharp Dell gets you:
LED backlight
Built-in Webcam + Mic
Built-in Speakers
MBP power supply
Nicer industrial design
^ If you don't need that stuff (or you want direct HDMI), by all means, get a Dell. Hell, I just ordered an Ultrasharp on Monday. But ACDs are still as good a value as a Dell.
Don't forget you also get a mirror with the Apple version.