I personally use a combination of both the midi keyboard and QWERTY keyboard. Also mouse editing of course for fine tuning. I find it essential to use the actual keyboard if your playing stuff that needs pitch bend/modulation, variances in velocity, just stuff that should sound natural and non-quantised. Stuff like hiphop, jazz. especially, where you need that human touch. It's also easier to play chords if you got a keyboard. However, for more trance, house type stuff, I sometimes just get away with using just the QWERTY and mouse, I get lazy to hook up my keyboard.

So I would say it's pretty important to get a midi keyboard. Your supposed to PLAY music and feel it. Your not really feeling the whole vibe if your painting in notes with the mouse.