I've never used them, however I've heard that they are very good. I'd suggest messing around with some of the examples (Fluid Effects > Get Fluid Example...), like the storm one.
I'll tell you this though, I have an instructor who used them (along with other software too like realflow and lightwave stuff though) on a couple of films (The Guardian, Pirates of Caribbean: AWE, Fantastic 4). I'm not sure to what extent nor do I have any insight into his process so take it for what its worth