All LCDs are not created equal. There are half a dozen panel types floating around out there, and different ones will produce different pictures and different results. Given the frequency with which "what monitor do I buy" threads pop up, I figured I would share all the knowledge I have on this topic.
There are four different types of LCD panels. For our purposes, we can discard discussion of TN panels because they are the worst of the bunch. We can also group P-MVA and S-PVA into the same group because they are very similar. This will mostly be a discussion of the differences between S-PVA vs. S-IPS technology.
Common Misconceptions
The panel in a Dell or a Samsung is not "identical" to the panel in an ACD. Do not worry, I used to think this myself. There are significant differences between the LCD panels that all LCD monitors use, and these different panels will produce different images on their screens.
S-IPS vs. S-PVA
S-IPS panels maintain their color accuracy very well, unlike S-PVA screens which begin to exhibit unacceptable gamma shifts if the image is only a few degrees off center from the viewer. This is especially important for any monitor over 20" because the color shift is so extreme that the image on screen will exhibit different colors in the corners of the panel. This is obviously completely unacceptable for a creative professional.
As a result of their lack of gamma shifts, many S-IPS panels are SWOP certified. A SWOP certified monitor can be reliably used for soft proofing your work, which has many benefits.
The downside of S-IPS panels is that they are more expensive, and also have difficulty producing higher contrast ratios.
Thats where S-PVA comes in. S-PVA screens can product very good contrast ratios - theoretically up to 3000:1, almost as good as a Plasma TV. They are also slightly cheaper to produce.
The downside to S-PVA panels have already been mentioned: unacceptable gamma shifts when viewing the screen only a few degrees off center. This makes them unacceptable for any kind of professional graphic design work.
What Panels in what Brands?
Modern Apple Cinema Displays
always use S-IPS panels. Modern ACDs are also all SWOP certified. If you are a creative professional, this is almost certainly the way to go.
All Samsung monitors use either TN or S-PVA panels. This make them very good for gaming and movie watching. If you are a casual, entertainment-minded user, then a Samsung is probably your best bet.
Most Dell's use S-PVA as well, but don't usually have the brightness or the contrast ratio that Samsung's have. If you are trying to save a couple bucks, then a Dell is probably a good choice.
A notable exception to that rule is the
older 30" Dell, which uses S-IPS. However, I am not sure if the older Dell 30" is SWOP certified. It probably is, but with Dell, you never know. The
new 30" Dell sports a higher contrast ratio and a price drop, which
may indicate a switch to S-PVA. As far as I can tell, the TFT panel in the new 30" Dell is unknown.
The Bottom Line
Different monitors are good for different people. All monitors are not created equal. For creative pros, or any field where you are doing color-critical work, an ACD is probably best. For entertainment-minded consumers, a Samsung is probably best. For tight budgets, a Dell is probably best.
Resources
WSGF's Master Monitor List. An impressive, comprehensive list of many TFT LCDs and their panel type, as well as contrast ratio, maximum brightness, and some other information.
Wikipedia's article on TFT LCD panels. As always, useful information from Wikipedia.
A thread over at MacRumors very similar to this one.