The actual answer is "it depends". If you have an iMac that came with a basic processor (like an i5) and want to upgrade it to a faster one (like an i7) then the next step is to check for an i7 that uses the same socket as the i5 that came with your system. The early/mid 2011 i5 iMac 27" uses an LGA 1156 socket, so it can be upgraded to an i7-860 which uses the same socket (2.86GHz quad core using 95 watts).
So you really have to do your homework and shop aggressively before making the decision on whether to jump into it or not.
The actual answer is "it depends". If you have an iMac that came with a basic processor (like an i5) and want to upgrade it to a faster one (like an i7) then the next step is to check for an i7 that uses the same socket as the i5 that came with your system. The early/mid 2011 i5 iMac 27" uses an LGA 1156 socket, so it can be upgraded to an i7-860 which uses the same socket (2.86GHz quad core using 95 watts).
So you really have to do your homework and shop aggressively before making the decision on whether to jump into it or not.