I was running a Q6600 based computer that I had for almost 4 years but just switched to a 2500k system and am pleasantly surprised at the boost with general productivity and the games I play. The new system is noticeably more responsive for me, for examples, iTunes no longer takes 10 seconds to start. This isn't due to the fact that I did a clean install of Windows on the new system; the Q6600 computer was never as snappy even after the periodic clean install. As for games, I'm not a FPS or twitch gamer and the games I play - Civ 5, Guild Wars, Dawn of War 2, Starcraft - ran well on the Q6600, even with my AMD 5850 video card at 2560x1600 resolution, but there seems to be a real, non-placebo improvement in responsiveness with the 2500k system (video card has stayed the same). Again, everything is just more snappy and I appreciate that. I got to beta test Star Wars and am currently beta testing Diablo 3 and I think they run much better with the 2500k system at the 2560 resolution. I also think I will upgrade to the next gen video cards coming out early next year and suspect much of the benefit would have been lost with the Q6600.
I considered waiting for Ivy Bridge but decided it didn't make much sense for desktop systems. Clock-for-clock improvement over Sandy Bridge don't appear to be that large, especially when one considers there will be a price premium on Ivy Bridge CPUs and motherboards for a while. The significant integrated graphics improvements with Ivy Bridge are more relevant to notebooks, where I think the case to wait for graphics and integrated USB 3.0 is more compelling - I'm thinking of Apple notebooks - than for desktops with add-in graphics cards where the integrated graphics would be bypassed anyway. Besides, I did a little future-proofing in case Ivy Bridge turns out to be worthwhile by getting an Asus Z68 motherboard that supports PCIE 3.0 and will be compatible with Ivy Bridge with a BIOS upgrade. Everything that will be in Ivy Bridge's Panther Point chipset appears to be already available with Z68 motherboards.