Since I trade NQ i rarely trade QQQ but because I often trade QQQ options I keep QQQ quotes from Island on my screen. Rarely do I see the market anything more then 3 cents wide and the times I have have traded Q's on Island the fills have been instantaneous. Cincy which is pretty much owned by the CBOE (regulatory reasons, thus the irony that Cincy is based in Chicago) is partnered with Island.
the options on the QQQ's are extremely liquid, and rarely have I seen a large opening gap on the QQQ's. As far as daytrading is concerned, I would think there would be better opportunities in individual stocks or in the futures, but that's not my specialty at the moment.