unreliable platform, poor phone service
I opened an IB account as a backup to my TradeStation account.
I only stayed with them for a month, I found the trading interface unreliable - when the platform let me down and I needed to get through to them on the phone hold times of over an hour were common.
The platform seemed most unreliable during fast markets or major news announcements... whenever the market was doing something interesting IB went down.
Fee's are cheap, and platform access is only $10 a month which is discounted if you do more than that in commissions. Cheap and cheerful, but you get what you pay for in my opinion, and when its my money on the line I want to use a more reliable tool for my trading than IB.
My experience is based on very short term trading, for which the level of reliability was unacceptable. Obviously if you trade longer term then these factors may not be such a big deal for you.