I use the Impulse 61 as part of my live setup with MainStage. All the faders, buttons, knobs and pads map easily to MainStage.
A minor gripe is that in order to use the transpose feature you first have to press both transpose buttons then use the knob to the right of the display to change the value. So what should be a quick left hand affair becomes a much longer sequence. If you play live and have to change keys of a song on the fly, you'll understand why it should be as quick and with as few steps as possible.
A major grip is the action of the keyboard itself. The reviews I've read dubbed the action better than the SL MK II and that piano players prefer the Impulse keys. For me the action is not even close to semi-weighted. The M-Audio Axiom has a better feel than this and the Axiom isn't exactly a stellar feeling keyboard.
Also the Impuse keys itself are a few centimeters shorter than standard keys and you'll definitely notice it right away. Its not mini per se but its like an airline trying to squeeze just a little bit more states into their commuter flights. You know somethings different but just can't quite put your finger (pun I know) on it.
If you're looking for a controller that integrates with MainState this totally works, just know the keyboard action feels cheap. Given the price, I guess that's to be expected.