I'm just working with Glu3D (http://3daliens.com/glu3D/index.htm) Really easy and fast to use. The PWrapper bit is well worth it too - basically and Max particle system can be 'wrapped' in a metaskin and it's really fast in viewport and to render. And it's cheap!
I had Realflow a few years ago at my old company and found it a bit daunting and involved to use and much harder to get good results.
Gizmo Animation Limited
Now i have tested Glu for some time and RF also. It seems that RF gives more "perfect" output visually to me but needs really fast machine.
I tryed to ask from NL if it will suppport more than 2 cores on workstation but no information on that.