Broadwell based iMacs will likely land around late-Spring or early-Summer due to the delay Intel announced not long ago. From what I've seen this should be a bigger upgrade (Haswell --> Broadwell) than Ivy Bridge to Haswell.
It really depends on if you need a system immediately or not or if you can wait 6-9 months.
If you can wait I'd recommend holding off for a few reasons:
- already mentioned a bigger performance jump from generation to generation with Broadwell
- will be outfitted with a better GPU at that time which may be beneficial if you ever go 4K, since that who "revolution" is about to land over the next few years
- fusion drives may either be faster (pretty fast today, not saying they are slow) OR SSD/PCIe storage is going to drop a lot more in price over that time
- might see an upgrade to the display panel built into the iMacs (big maybe)
- TB2 although I don't think that's really critical unless you have a really specific use case
Just my opinion/thoughts. If you can get by, wait it out, otherwise the Haswell iMacs are pretty nice. I'm typing this from one now.
