Things I liked: Gives you 20 shots instead of 10 before you have to reload.
Things I would have changed: Add an indicator showing when each of the two mags is empty. (You do NOT get 20 uninterrupted shots: you get 10, then have to manually shove the second 10-round mag into place.) This is one expensive item!
What others should know: I occasionally get two shots from one trigger-press, unless I am very deliberate about the trigger press.