buy now, not for performance just to get one,
15% isn't that much i mean you could say for years "I'll wait"
but then you lose your job and go "I wish I bought 2500k when i had money =("
and that'll be when you actually need the upgrade!
I'd go with the 2500K now. Ivy Bridge is mostly just a die shrink and optimization in terms of raw performance numbers. And like 'horney' said, you could play that "I'll wait" game till the end of time. Go with the 2500K now, and you'll have a good buffer to justify upgrades when the whole new Haswell architecture rolls around in 2013, which will assuredly provide a greater that 10-15% increase over Sandy and Ivy Bridge.
Agreed, very valid point that ivy's performance gain surely won't justify the price increase at least it should be more when it is released.. Plus the drivers and everything for sandy bridge has been tweaked and adjusted over time. My personal preference would have to be the 2500K sandy bridge Overclocked of course
While Ivy Bridge's aren't slated to have much of a bump in direct performance, they ARE however supposedly going to use much less power and run cooler. Many say they will be great for increasing laptop battery usage. However, if they run cooler, this only means you can then overclock them to higher speeds than the i5 2500k and thus indirectly eek out even more performance than before.
They aren't going to be particularly more expensive so it might be worth the 2 month wait. Nonetheless, the i5 2500k is very good. If it were an 8 month wait I would go with the 2500k, since it's only a 2 month wait now I would say at least consider waiting.