I have always found StockXpert's site to be fast and efficient; a complete contrast to Fotolia for instance. All credit to StockXpert for having the necessary technology right from the start instead of doing it on a shoestring.
Similar comments apply to Corbis - although the site is at an early stage of development, it is fast and efficient and they've got very good communication.
Investment and professionalism is so important in this game, and that applies equally to us as photographers of course.