Intel's Core i7 line of processors are marketed towards the business and high end of the consumer market. This differentiates them from the i5 processors, marketed towards the main stream consumer market, and i3 targeted at the entry-level consumer market. While the i7 processors are quad core processors, they feature Hyperthreading technology that makes them appear as 8 cores to the processor, so they can handle 8 simultaneous processes.
Personally making the transition from Intel Core 2 Duo quad core to Intel Core i7, the transition is just as apparent as from my previous transition from one to two cores, and from two to four cores. However, while I sometimes find with the Intel Core 2 Duo quad core things would sometimes slow down to a halt, I haven't yet experienced this with the i7. I can really challenge the i7, running several demanding applications and processes simultaneously, without so much as a temporary slowdown. I love being able to initiate a demanding process (like rendering) in one window, then jump over to another to entertain myself while I wait for the process to complete. This is something I can also do with the Core 2 Duo quad, but the difference is, sometimes the Core 2 Duo quad dislikes the windows swapping and slows down everything for a bit.
The Core i7 is a perfect match for Windows 7 64-bit. Use this with at least 4GB of high speed RAM, and you have a monster system at your hands. As mentioned in the introduction, mainstream PC users should be fine with an i5 or i3. However, for power gamers or those that are into heavily demanding processes, such as audio recording and production, 3d design and high definition video work, the i7 can really be worth the investment.
This is a processor that should last several years and will serve you better in terms of performance and longevity than an i5 or i3. However, mainstream users on a budget should look at the i5s and i3s.