***Update 06/03/2013: The next generation Haswell i7-4770K and i5-4670K have superseded the IvyBridge i7-3770K and i5-3570K. They are between 0-15% faster in most regards, depending on task. See AnandTech's "The Haswell Review" for details.
As of 04/29/2012, this is the current top-end "premium" tier Intel chip - about 5-10% faster than the i7-2700K which it replaces. The current top-end "extreme" tier is and will remain the six core i7-3960X until the second half of 2013, when the Ivy Bridge-E is released.
It overclocks nearly as high as Sandy Bridge and heats up more with voltage. This means 4.4 to 4.7 GHz will be around the limit on air. Note that if you do not plan to overclock and/or intend to run virtual machines, the plain 3770 is the cheaper and better choice as it has Intel SIPP, vPro, VT-d and TXT enabled (the K has these disabled).
At its official retail price, three hundred and thirty two dollars, it is currently one of the best values for a high performance chip in the market. The next steps up are 2-3 times this price.
The GPU performance compared to the 2700K is about 50% faster, which is equivalent to a $40-60 video card. This is enough to play most games at mediocre quality with a mediocre framerate or run Quick Sync very fast (Intel's custom medium-quality h264 encoder; about 300 frames per second on 1080p video).
If you do not have much use for the GPU, most i5 and i7 owners will have a hard time justifying the Ivy Bridge upgrade. All other slower chips will see a substantial improvement. Another good value is the Ivy i5 3570K which is something like 0-15% slower and 30% cheaper.
The new motherboard lineup consists of the Z77, Z75, H77, Q77, Q75 and B75 chipsets. The major improvements over the Sandy Bridge generation is native USB 3.0 support, PCIe 3.0 (with Ivy chips only) and SSD-HDD hybrid caching. Z/H/X all have CPU overclocking. The 77s have the SSD caching. Most owners of this chip will probably get a Z77 which is feature rich and nearly the same price (about ninety dollars for the cheapest board currently).
[Sources: AnandTech, TomsHardware, overclock.net; see comments for some minor extra detail]