The Antec CP-850 is groundbreaking in a number of ways. First of all, it's designed specifically to be used with an Antec 1200, Antec P183 or Antec P193 PC case. If you don't have one of these cases, don't buy this PSU--it simply won't fit. And therein lies the crux of what makes the Antec CP-850 different. Unlike other power supply units, which have been about the same size and form factor since the IBM PS/2 was released over 20 years ago, the Antec CP-850 forgoes any attempts to cram high performance power and cooling into a tiny PSU by taking on the CTX form factor. The Antec 1200, Antec P184 and Antec P193 cases have larger housings for the PSU to sit in, meaning that it's easier for computer builders to configure the PSU and hook up cables.
The size of the Antec CP-850 may also make it easier to manufacture, which accounts for its lower price tag. As far as 850W PSUs go, the Antec CP-850 is a fraction of the price. Comparable ATX power supplies cost 50 to 70 percent more than the Antec CP-850.
Another benefit of the CTX form factor is more efficient cooling. The Antec CP-850 uses a 120mm PWM fan that blows directly across the module, rather than at a 90 degree angle. This greater efficiency allows the system to run much more quietly.
If there are any cons to the Antec CP-850, other than the fact that you'd have to upgrade to a new case, it's that the PSU isn't very modifiable. Replacing the fan is difficult to impossible, and out-of-the-box, it doesn't come lighted. While this latter aesthetic quibble may be a non-issue to you, to hardcore PC builders, the inability to tweak and customize may be a drawback. Some of the wirings are only semi-modular, as well, menaing that some harnesses won't click in fully, though you'll still be able to connect everything you need (motherboard, video card, SATA drives).
Bottom-line: the Antec CP-850 makes sense. It's a natural evolution for power supply units that matches the escalation of CPU and GPU capabilities. A larger CP-850 is easier to cool and easier to configure out of the box--yet for some reason, PC case manufacturers seem to be stubbornly holding on to the ATX standard. Given that most gaming PCs require at least a 500 watts, while a SLI/X-Fire setup can consume even more, it makes sense to leave some leeway for a larger PSU.
Just remember: You're best off getting one of the compatible Antec cases for this PSU. While it may be possible to fit a CTX PSU into a different case, it would require much finagling and would be an ungraceful solution at best. When paired with one of the abovementioned Antec cases, the CTX's form factor is a definite plus--everything fits together and integrates perfectly.
So, if you're in the market for a new PSU and a new PC case, consider this winning combo.