Even though a new one might be released soon after you buy one(if that's the route you decide to take) I seriously doubt that you'll have any kind of buyer's remorse.
I bought my early 2009 iMac less than 2 months before they updated to the 21.5" model and I still have it and love it. I've used it for iBooks, trying out Geek tool stuff, Virtualizing different OSes and using it as my main computer until February of this year when I got my MBP. Nothing has slowed down on my iMac, but I needed something smaller and more portable so that I could work while also watching my kids.
If you really want one, get one. Or if you want one and want to save money by buying one that is still very capable and a previous generation; buy a refurbished one.