i had an account with izone ($5 / trade, AMTD subsidiary). same everything as ameritrade, except they don't give you the toll-free phone number. i used it anyhow.
ANYHOW...here's a couple reasons not to use them. these have happened in the last year:
-> one of the things i do is trade some of the cheapie stocks. so i've got a large position (12k), and want to cover my short. there's 25 or 30k on the offer, i put my buy cover in - let's say at 76c, which is where the offer is - and level 2 pops up with my size (12k) buying at .75. i cancel, it disappears. i do it a couple more times, same thing. other people start lifting the offer, i have to eventually put in an order with limit 5c away (.83) and get filled at like .81 eventually. the process took 30+ minutes of a stock that had volume. when i called and complained, they said on cheapies, AMTD doesn't guarantee your order will be shown.
-> placed a market order to fill on an NYSE stock which was active. watched as 40-50k worth of trades were done. i only had a few hundred shares. my order took 5 minutes to be filled, in 100 share increments, about 20-30 seconds apart. one time, they offered me free trades; the other time, since the trade actually went in my direction (!) they said nothing could be done.
the site goes down more often than my direct access brokers as well.
i moved my money out, and won't go back. the cost of their cheaper trades is more than offset by the periodic problems like these.