Personal experience suggests you cannot go wrong with punching a refurbished iMac from Apple, I'd caution to wait until new 2012 model is released though as highly probable you'd actually get a 'new' as opposed to 'used' model - be it 21.5 or 27in form factor.
On the plus side, and as many have noted on these boards, you may have upgraded iMac via refurbished route, i.e., bigger HDD/ more RAM, upgraded RAM GPU Ram memory in 27in model.
On the negative side, you get a lemon, specifically one with LCD issues, which means testing properly yourself and swapping if any issues are apparent.
Further, on refurbished models you have same 12 month Apple Warranty and ability to add AppleCare. Indeed, by not purchasing new and adding AppleCare, whilst paying only a fraction less than new, you do then have a full three year warranty with the added bonus of knowing should you sell on again, you'll get a greater return for a used Apple with APP than without, and say within a two year timeframe, with funds from selling your used Apple, you can then re-invest in another refurbished one.
Look at it in cost of ownership terms by dividing actual cost with number years you intend to keep the Apple - the refurbished route offers good value for money, particularly given this years update will not offer a significant boost in speed terms - maybe in GPU terms - iMacs have never been brilliant games machines, but as part of a media centre set-up/ pure computational uses, they really are good value and aesthetically look nice too - best all-in-one on the market in my opinion.
