Quote from OptionsWizard:
The way Ib deals with interest is very transparent on their site and very fair in my mind.
Furthermore, the interest you get on your funds is very high. I don't know of any other broker that gives you higher interest.
That said, I tried to trade FOREX with IB and didn't like the way IB handles the accounting side (maybe I couldn't understand it fully). Long story short, I moved to trade currency futures on CME.
Liquidity is more than enough. The spreads are as tight as FOREX (1-3 pips) and these futures are integrated much "cleaner" in IB environment. For example, there is no issue with interest because it is priced in the future contract.