The rankings are off just like the other tools but it gives you a nice starting point.
Its just one datacenter, and it seems to me like its always the same one! You will always find different results in different datacenters, generally a little better!
The most important thing, to me, is that Google frowns on the automated rank checkers because they waste bandwidth. I am not sure how they catch you but if you have the Google toolbar and analytics, I think they must be able to tell if you use the rank checking programs and you will be penalized. Just my guess but that's all anyone has with Google is guesses.