The Ivy Bridge CPUs were limited by Intel's new fab process which resulted in much higher temps than with Sandy Bridge for the same clock speed .... hoping that Haswell is not limited in this respect.
-The value of an IB system will drop as soon as Haswell drops
-Haswell's new socket requirement means the MoBo you buy today will limit future upgrades.
-Reported 7 - 12% performance increase clock for clock
-Reduced power and heat
http://www.tomshardware.com/reviews/core-i7-4770k-haswell-performance,3461.html
http://www.extremetech.com/computing/150999-haswell-leaks-significant-performance-boosts-mixed-with-odd-design-decisions
On the other hand, the production line will still be in its adolescent stages meaning the OC success of individual CPUs will still vary wildly. As the production line matures, this variability tends to shrink. In addition, remember on January 31, 2011 when Intel announced the detection of a design error in the new Intel 6 Series support chipset ? Everyone with the B1 MoBos had to wait for B3 designs and rebuild their boxes or limit themselves to the SATA 3 Gbps sockets.
Personally I prefer to stay off the "bleeding edge". While you lose the opportunity to be the 1st one on your block to have the latest and greatest, your chances of getting a good overclocking CPU improve, you lessen the chance of a design problem, and you also have access to the "new and improved" versions of MoBos.