Their execution is flawless. If the order is eligible to be filled... instantaneous. Great fills. It's quite an investment in time to learn TWS (their trader workstation software) completely and therefore use it effectively but it is also well worth the effort (an initial futures trade should never be submitted as anything other than a limit bracket order). I love their commissions and their policies. I really only have one complaint - for options traders the tools are not there like they should be, that said I still use them for executing options trades and supplement with external tools. And forex, you see the real spread, no manipulation like so many others.
