BEST ECN, when it's properly working, is very good. Unfortunately, there still seem to be some bugs that need to be worked out there. As far as a price gap between Best ECN and NASDAQ, it all depends on that moment. If the inside bid and offer are both ECN's, then at that moment there is no price gap at all. If there is a market maker at the best bid and and ECN a few levels down from there, then there would be somewhat of a price gap.
As of right now, I'm still using ISLD to execute the majority of my NASDAQ trades. As soon as I see BEST ECN get ironed out, I'll use that as my primary execution route.