it sucks. it's an "invisible auction." it lets institutions see your order flow and gives them a chance to take the trade before ECNs see the order. so they get to see your hand without showing theirs. primex is a black hole for MMs to hide inside and buy/sell without disclosing their interests. i'd recommend avoiding it, even if only on principle.
I heard 75% of what trades on Primex is listed stocks. Is there any improvement over the specialist? Ciao...
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.