I tried the Amazon EC2 cloud and have to say that I wasn't very thrilled with the experience. The instances were low powered and naturally rendering was really slow. The instances with more CPU-power caused problems and couldn't be started even though they are included as options in the submission ROP. That was H14 though, maybe this has changed with the current release. Another thing that really bothered me is that you are charged by the hour. With EC2 your renderings are downloaded automatically. While this is a nice feature I had the problem that my home machine crashed during the loooong rendertimes. Thereafter I couldn't reconnect and my renderings were gone. Take this experience with a pinch of salt though as I didn't look for a solution. Maybe there is the possibillity to reconnect, get your files via ftp or so.
I also tried out the Gridmarkets solution which I found much more user friendly. You are charged by the minute, you are not charged for up- and download times/bandwith, render machines are faster and you have ftp-access to your renderings. Another thing that happened to me was that I didn't set my output paths properly (because Gridmarkets didn't support $JOB at that time) and therefore rendered into 'nothingness'. Still I was able to access each frame through their web interface and download it manually. Not very professional but better than to rerender everything.
I didn't do any cost comparison between EC2 and Gridmarkets though, as Gridmarkets offered some free credits for beta testing