Well firstly, it's not cheap per se, but the amazon price was the best I could find. It's performed faultlessly so far.
This is far more power than most systems will ever need, but anyone looking at a dual graphics card setup, especially an Nvidia SLI one, will want to be sure they have enough headroom to handle high load levels.
Anyone looking at a 'normal' desktop build, or a single GPU gaming machine should probably look for something in the 500w-600w region and save themselves some money.
The modular design is great, it's quiet, and it's understated unlike some of the competition. There probably isn't a brand in the PSU market with a better reputation than Corsair and this PSU should outlive almost all of the other components in a system and the 850w should provide adequate future-proofing for many years.
My spec for reference (I doubt I'm using much more than 600w of the 850):
Intel i5 2500K @4.4Ghz
Gigabyte P67A-UD4 Motherboard
8GB DDR3 RAM @1600Mhz
2x HD6950 in crossfire
2x 1TB SATA HDDs
4x case fans with LCD fan controller.