If you make your living using your mac, and the productivity increase of the new model more than outweighed its cost, then yes. Otherwise, no; keep using it until it no longer meets your needs.
My needs are simple, so a 20" 2.16 c2d white imac was working just fine. I recently got the base 27" iMac mostly due to the increased ram (vmware would gobble up it's 3 gigs), gorgeous display (and the ability to run two more) and thunderbolt. The white iMac is going to an employee whom I'm trying to wean off of windows.
My needs are simple, so a 20" 2.16 c2d white imac was working just fine. I recently got the base 27" iMac mostly due to the increased ram (vmware would gobble up it's 3 gigs), gorgeous display (and the ability to run two more) and thunderbolt. The white iMac is going to an employee whom I'm trying to wean off of windows.