In my experience with Nasdaq stocks, PRIMEX is bad news. It basically allows MMs to delay your order while they take a look at it. The problem is, if they decide they don't want it, they cut it loose and the market has moved away from you by then.
IB included it in their BEST/SMART routing for a few days, and it was terrible. For the way I trade, PRIMEX offered absolutely nothing positive, and created serious (expensive)problems.