The 27 inch iMac has been benchmarked as the fastest stock Mac ever, according to MacWorld tests.
Our tests of the built-to-order Core i7 iMac (which, other than the processor, has identical specifications as the stock Core i5 iMac) showed even greater performance prowess. With a Speedmark 6 score of 225, the $2199 Core i7 iMac was nearly 8 percent faster than the Core i5 iMac. The Core i7 was nearly 11 percent faster than the $2499 2.66GHz Quad-Core Mac Pro and 9 percent faster than the 2.26GHz 8-Core Mac Pro, which sells for $1100 more. In our tests, there were a few tasks where having eight physical processing cores was beneficial, like our MathematicaMark and Cinebench CPU tests.
If you don't need the expandability of the Mac Pro it seems the iMac is finally a great option for those who need the performance.
well, it's way faster than my 13 Macbook Pro for sure. But it's probably a bit slower than my water-cooled Core i7 at 4.4ghz with dual ATI 5870 in Eyeaffinity mode. Probably and just by a teeny bit because it's always hard to tell with the superiority of Mac OS. Then again...