Last I checked, their commissions were way too high for their service to be seriously considered by most active traders.
Also, you hardly ever see anyone on ET who isn't exclusively forcused on equities or equity index instruments, though there do seem to be a few people trading currencies, and I seem to recall the brief eruption of an oil futures thread. Nary a mention of grains or bellies or beans... For most of us, only the equities-oriented brokerags or prop firms are relevant. IB may be a special case, as a brokerage, but not in terms of its ET-using clientele.