I bought the low-end iMac 21.5" refurbished. I was seriously considering the iMac 27" with either the i5 or i7, but decided against it after reviewing my actual computer usage and the rate in which I upgrade. Why pay more for technology that will last 6 years when I am planning to upgrade in 3?
I had major problems with kernel panics, but it's been a great machine after a logic board replacement. No regrets whatsoever in getting the "low-end" iMac.