I traded futures (mainly ES, but also ER2 back when it existed, and just a little NQ) for a couple of years with MB Trading.
You're referring to the MBT Navigator, when you refer to live quotes? As far as I could tell, the quotes matched right up with my eSignal, and I was able to scalp trends (going for 1 point initially, 4 ticks, on the ES, then moved to 1.75 points, 7 ticks) (for the ER2, I think it was 2 points, 20 ticks, was it?).
There was slippage on stops, but that was always because of market volatility, of course. The data feed was solid.
As a side note, I've since switched, to take advantage of lower margins (though the MB commissions are the lowest I've seen publicly quoted).