Advantage Futures (
www.advantagefutures.com) is pretty big and very well capitalized. Not quite as well as Man or Rosenthal, but if you look at this list from 2004 (I'm sure a more recent one exists, but I just did a quick search),
www.futuresmag.com/reprints/218866.pdf, you can see that they're just a few slots lower than Interactive Brokers. Plus, they're clearing members of 4 different exchanges....which in my mind makes then pretty credible.
As far as the commissions go, I'm not certain what their rate is without volume. I don't think its anything special, plus you get zapped with X_TRADER pro to the tune of $1250/month. However, if you do volume, or you keep a ton of cash in your futures account and want the peace of mind of feeling as though its pretty safe, I personally think Advantage is the best and would probably not trade anywhere else (although, as I said, I do trade with TransAct and think they're a great operation as well).
As far as their technology goes, I have a buddy who runs a trading arcade. He told me this story about this time that Advantage called him up one day out of the clear blue to let him know that it looked to them that there was a bunch of noise on their connection, and that it had been gradually getting worse throughout the day. He told all of his traders to think about closing out any big positions, and 20 minutes later their connection went down all together. He said that he never would have expected his FCM to notice something like that, but claims that they saved him and his traders a ton of money just from being proactive. I know for a fact that Advantage has a lot of traders who love trading with them for all sorts of reasons like this.