The SuperSOES info. is correct (reviewed by NASD), except that a new fee structure is phasing in. Fee pass-throughs are per broker.
I am with Magna on this one. Here is a real world example: This morning (11/21) I shorted ADVP on a gapdown, why, cuz I saw the # INCA on bid and offer is off balance (#on offer was way more than # on bid), that's how you use L2 to decide your Entry/Direction... Even you swing trade, L2 can help with your entry by giving you an instant cushion (go your direction for .3 or more), then you wont get stop out at your entry... Just my 2c Good Luck