ITreviewed posted a review on the Intel Core i7-975 Extreme

Intel's Core i7-975 Extreme Edition is an insane processor. Not only is it the most powerful desktop processor ever, its 3.33GHz quad-core performance can be increased to over 4GHz in a matter of minutes thanks to the unlocked multiplier. You're going to have to dig deep to justify the cost
compared to the fantastic 2.66GHz Core i7-920 D0 Stepping (from =A3215/$319), which can also be overclocked close to 4GHz, and you'll need applications that require four or more processing threads for it to really shine. But if you are after the ultimate in performance and price isn't an issue, the Core i7-975 Extreme is the processor for you. For the rest of us not planning to set a new world record, you don't need the Core i7-975 Extreme - the Core i7-920 remains the best balance between value and performance.

Intel Core i7-975 Extreme reviewed