I agree with UTclassof89 - I've been using it on and off since early beta version. Very useful.
As long as it's free, it's a very good alternative to quick and dirty testing which I previously would have used a service like browsershots.org or NetRenderer for.
Obviously it's no substitute for a virtual machine with the actual browser you need on, or a PC with IE on, but I have those separately for more in depth testing when needed.
Until I have to fork out for the subscription (which I will have to at some point if I want to carry on using it) it gets a big thumbs up from me!
/Doug