I am currently with Sharebuilder, and I'm looking for a new broker for... reasons.
First is they have no trading tools whatsoever. They have charts, but at a 15 minute delay, it's better to look elsewhere.
Commissions are pretty crazy. They charge $6.95/trade with $0.007 per share under $1. The commission maxes out at 15% of the total trade, which makes it quite hard to trade pennies effectively.
Order executions are extremely quick, usually within about 30 seconds of placing the order, I saw it pop up on the stream.
I originally got this for trading NASDAQ and other higher stock prices, but you can't beat the profit margins of pennies. Sucks I didn't realize the fees until I already made the account. Such is life though I guess.
Hope this helps anybody interested in Sharebuilder since I'm not seeing much information on the intro.