I have received 10 fills from PRIMEX today (out of about 200). The stocks were NTAP CSCO VRSN INTC AMAT. I think that they are only routing to PRIMEX for the NAS100 stocks, so you might not see it for other NASDAQ stocks. I almost always use limit orders, so I can't comment on market orders.
I have noticed a slighting sluggish response with BEST_ECN on some fills this week, but not sure if it is IB's quotes or maybe I just missed hitting the bids.
I would like the option to disable PRIMEX routing because it appears to have issues versus real ECNs like ISLD, INCA, and the owners of the exchange appear to be anti-ECN market makers.
Fleance