I also have an account with them and have been overall unimpressed with the platform. However, their commission structure is excellent for an SPX trader like me because of the flat rate pricing. Given the exchange fees of .44 per contract on the SPX, broker fees, clearing fees, etc... they must be losing money, particularly on index traders. A 100 lot SPX order costs them roughly $50 and they're collecting $10. That's the one fear I have about the firm: they have an unsustainable business model.
They clear Penson, who also clears for TOS, so I'm not sure why they would have another firm for PM.