This site would be good for doing a quick check while coding css, to make sure everything is proceeding nicely. I did get some strange errors and anomalies when testing one site that would cause me to do further testing before actually using it in production. Some of the errors looked like a problem with the application itself (some Windows pop-up dialogs in German), and others showed problems that I know don't exist, like missing images, that area probably due to slow download of the application. In fact it took a good 4-5 minutes for my 30+ browser images to load, and I was warned it could take up to 35 minutes. Of course that's the free version, and you can purchase priority processing for a monthly sum.
If you are willing to shell out a monthly fee, the priority processing might make browser shots work for you. The problems I encountered with the free version would make me reluctant to trust it's results. Have a different experience? Let me know what you think.