It depends what you need it for. That's roughly $200 cheaper than the low end 21.5" of the new ones. If you don't need the newest of the new the 2011 models are great computers.
The reason I say it depends what you need it for is because, as far as iMac's go, the baseline 21.5" is a little on the weak end. Plenty if you want just basic stuff but maybe not powerful enough if you want it for high end video or sound work.