If you are building it right now, you will have more confidence going with Ivy Bridge. Haswell motherboards, especially feature-rich ones, have not been tested/used much and there is always a risk they have bugs (still relatively low risk).
Performance differences will matter less depending on what your workloads are. You will be paying less for the ivy bridge parts once its corresponding haswell chip is released.
Ivy Bridge CPUs will likely be marginally less expensive when Haswell is released. For example, based on Newegg prices, the i5-2500k was selling for $225 prior to the release of Ivy Bridge. After the i5-3570k was released, the i5-2500k dropped to $220 excluding any sales / coupon codes.
Personally, I am not sure if I will upgrade to Haswell or Broadwell yet. Either way I will be upgrading to Skylake / Skymont when that is released. Haswell / Broadwell will then go into my HTPC.
So. Seeing the early Haswell benchmarks out of china leaves a lot to be desired.
Performing -less- than ivy bridge in some respects.
I'm building my first computer so this isn't an upgrade question.
But really, would later bios updates make haswell worthwhile?
Or is it worth spending the extra 40-60+ on Haswell and a 1150 motherboard...
I wouldn't be upgrading for years, either. if that helps.
I am waiting to see how Haswell overclocks (just rumors, but would be disappointing if you buy now and turn out to be true), and maybe, just maybe the they wouldn't change the socket again with Broadwell.