Intel is officially introducing their newest CPU lineup, codename "Haswell". Its the successor to Sandy and Ivy Bridge, and uses a new socket LGA-1150, instead of the old LGA-1155. This means they require new motherboards, not a drop-in replacement like Ivy was to Sandy.
The reviews are hitting the internet, the best one IMO is at the [H]:
http://hardocp.com/article/2013/06/01/intel_haswell_i74770k_ipc_overclocking_reviewHere's another one at a review site I respect quite a bit:
http://www.anandtech.com/show/7003/the-haswell-review-intel-core-i74770k-i54560k-testedTLDR: This is a minor, incremental upgrade to CPU speed over Ivy or Sandy Bridge. At stock speeds, you might see a 20% increase over Sandy Bridge, if you overclock, statistically less (highly variable based on your luck before and after though).
If you're using anything older than Sandy Bridge (discounting AMD options of course), then you have a good opportunity to upgrade. But you're nearly as well off upgrading to a Sandy Bridge bought second hand as you are upgrading to Haswell. I imaging the usual places will be flooded with used Sandy Bridge CPUs that can be had for cheap.
As for me, I'm getting a Haswell. But only because I want to upgrade my secondary computer. My "old" but still awesome Sandy Bridge i7 2600k was a relatively poor overclocker at 4.3, so I stand to improve my luck with Haswell, and I'll just relegate it to my secondary PC. Haswell will be my new primary, and my current secondary will be demoted to HTPC or just sold off. If I was just keeping one computer, I would skip this round just as I skipped Ivy Bridge.
By the way, I found the old Sandy Bridge thread on these forums, which is worth a laugh. I think what I said back then about them turned out to be pretty spot on, compared to most:
http://bbs.hitechcreations.com/smf/index.php/topic,303523.0.html .