I have noticed many of the old biggies (iStock, etc) are making lots of imporvements to their interfaces. I would have to say that iStock seems a lot slower now, but maybe they'll install more servers and speed back up.

CanStock has made major improvements that I think are really nice. I really like the series options that they have, allowing you to QUICKLY build series of images that are related and viewing interface is nice to.

my 2cents