I just transferred my account out of Mirus/RCG. I also looked at Dorman.
I don't have a problem recommending Mirus for most people, as long as you're not looking to do anything "complicated". Ie, if you're scalping a few ES/YM contracts... go for it. Mirus is a much better than average firm in that regard. And RCG won't get in your way.
I don't fit in that profile for several reasons. I was trading physical commodities, and also had specialized pit requirements. Mirus didn't have much expertise in these products, and couldn't/wouldn't help me with finding workable solutions.
I will say that while I have some remaining warm feelings towards Mirus... I am extremely disappointed in RCG as a FCM. RCG has an appallingly ignorant risk management department. Also be very careful if you go with RCG directly as your IB, because their ehelp application people are simply incompetent. (By going through Mirus, you get their expertise with setting up ZenFire + R|Trader.)
I really wonder how RCG recruited for risk/ehelp, two rather critical departments. Picked these guys up in the parking lot of a Home Depot?
Oh, one last gripe. Now that I'm on Vision's futures platform... RCG's "passport" website is incredibly dated. Vision's platform looks much, much sharper, updates quickly, has daily statements going back months, etc, etc...
Again, if your trading is simple, you'll be fine with any of these choices. But anyone trading options, trading more than 6 digits, or trading more than the indexes should look around carefully. I'm not sure that Dorman is too much better in that capacity... they're a very risk-averse small shop (< $50m in customer funds last time I looked). I believe they're not even clearing members of the non-Chicago exchanges.