I'll add a couple of notes about Rydex to the other two posters.
--If you hold money at Rydex, you can trade the dynamic funds twice a day. I think it is an hour or so after open and then at the close. QQQQ at a broker can be traded all day of course. Then again, you can only trade Rydex funds at Rydex.
--Rydex Velocity won't match 2X QQQQ exactly over time because Velocity is priced each day. In other words, you have compounding going on.
--Rydex lets you float a little, which is great if you have a system that has cash signals. Why is it good? Because you can put your cash in the bank or temporarily pay off a line of credit waiting for the next signal. When it comes, you can buy Rydex funds before the money gets to Rydex (assuming you have wiring instructions set up).
As an aside, Rydex has the best tax reporting of anyone I have seen.