I use it, and like it. Depending on your purpose, the account/transaction fees add up. It's $4 for automatic investments, $9.95 for market orders (buy and sell). Also, transactions aren't immediate for market orders, and often can lag by quite a bit of time. Also, automatic investments are only executed on Tuesdays.
Overall, it's very good for long term investing and dollar cost averaging. It's not setup for day trading. The fees for automatic investing are cheaper than most, but market orders are fairly expensive.