Haswell has already landed, and if you're building a new rig today, you might as well jump on the new platform. At the same time, Intel clearly put a great deal of focus on mobile friendly features, so even though Haswell is a step up from Ivy Bridge, it's not the end-all-be-all that some where hoping for. Haswell-E, on the other hand, brings the desktop back into focus and will offer 8 processing cores and 16 threads. That's not all.
According to our sister site TechRadar, Haswell-E is likely to wield hidden cores. Specifically, it will have 14 cores sitting underneath the hood. Imagine what you could do with all that processing power! Now stop imagining, because you won't have access to it.
Don't be too bummed, it's not as though the majority of applications out there can take full advantage of the processing power that's currently available, never mind a 14-core chip. And let's not forget that we're still waiting on Ivy Bridge-E parts, which will likely top out at 6 cores.