I was surprised myself not to see Datek listed. I've been using them for about 4 years now and really like them.
Last time I checked (and please correct me on this if I am mistaken) Datek owns the Island system.
In fluid/quick situations I have always had fast fills placing strictly only limit orders in these two ways:
1) buying on the ask
2) selling on the bid
When I first started I tried Etrade along with Datek at the same time. I cancelled Etrade after the first couple days of serious trading. I think they use the US postal system to place orders (or maybe Pony Express), Etrade is so slow and Datek so fast! (to me trading with Etrade is like using AOL to go online). Nothing will anger you more than trying to hit a fast moving target and having to wait until long after the move to find out you didn't get filled (Etrade). Datek is not like this at all, you'll know pretty quickly if yes or no and can respond accordingly quickly.
Datek has tons of other great features as well, such as their fantastic Streamer features: Streamer, Portfolio, Last Sale, Index, Level II, Actives, News, Depth Chart, Chart.
Their support people are also fantastic and very helpfull, always keep their number in your cellphone. They will help you in all manners, clear up issues, place trades for you if you are mobile, having system problems, etc.
Anyways,
go Datek!
Thumbs up to Datek