I like them because their margin rate is 4%.
Ameritrade was 8% and this saved me quite a bit.
Click here for my referral link.
The only downside is they aren't set up for Quicken yet, but I expect this to change. If you carry margin balances, then this is a great choice.
I've gotten over $200 in cash back from them at places I would have shopped anyway! Ebay, Staples, more! I put it at the top because they ROCK. Basically, you get a kickback from Ebates when buy.com or whoever pays them. Win-win. If you like throwing money away, don't use them!