I personally use a GTX570, and it is pretty decent, though not spectacular. Costwise, it is reasonably priced, and "good enough" for most of the networks I have tried (minus ImageNet...)
A key problem is limited GPU memory in the Fermi series, as it is difficult to fit a truly monstrous network on a single card. Krizhevsky's ImageNet work had some very tricky things to spread it across 2 GTX580s, and the training still took a very, very long time.
Thanks for the heads up friend!
Once again, my 570 is slower than a 580 (about 2x), but "good enough" for now.
You can check out the code here - it is really good IMO.