I made a blog post about it some time ago.
I own and run two myself, and find them very user friendly, and an engagement at 40ft with .20gs is not all that bad. They sound worse than they really are, but keep in mind using one at 30ft and under is going to be painful if not interpreted as malicious at best.
Take a look maybe what I've found results wise might help you make a decision.
http://schoolboyairsoft.com/40mike-airsoft-inovations/