It will be faster, of course --but it's not just the cost increase of the CPU, but the mainboard as well. When you compare predicted pricing of SB-E to sale prices on an i5-2500k ($189 every so often, with a lifetime low of $149 from Microcenter) or an i7-2600k (I got mine at NCIX-US for their grand opening price of $269), and then compare the likely board prices with mid to upper-level Z68 chipset 1155 boards ($180-350 depending on features), it's only a great deal for someone who is doing rendering duty, video encoding on a near-daily basis, or who absolutely has to have the best Folding farm on the planet regardless of cost.
It's not just that there isn't anything an i5-2500K or i7-2600K can't run --it's that there's hardly anything they can't run well. That, and the first Ivy Bridge processors are likely to be Socket 1155, giving a quicker upgrade path to those who stick to the mid to upper-mid end of mainstream.
Finally, PCIe 3.0 is nice if it happens to come with the rig you get, but it's not going to be necessary for probably the next two years, at which point we'll have Haswell to think about. IB processors are supposed to be compatible even with non Gen-3 boards, making that a non-issue.
Core i7-2600K @4.2GHz, GIGABYTE GA-Z68XP-UD5, 4x4GB G.Skill RipJaws PC1866
Corsair 650D case, Seasonic X750 Gold PSU
WD `Raptor 600GB boot, WD Caviar Black 1TB data, NEC 7200 DVDRW
2x Radeon 6970 2GB in Crossfire, X-Fi Titanium, Dell 2407WFP