I use the AX-Edge as a MIDI controller and its amazing for this role.
There is honestly no keytar out there that looks as cool as this one (and well, if youre out there buying a keytar, lets be honest, thats the most important aspect). The sounds are based on the Zen Core engine that Roland uses on most of their modern instruments and its a 50/50 if you like them. I do not. They tend to sound fairly round and smooth and I prefer more harsh tones.
As a MIDI controller its amazing and versitile. Everything can be mapped to any CC message and given that theres 3 buttons on the back, 4 on the front, a modulation bar and a ribbon theres alot of possibility. (Tiny tip, set the mod bar to sustain, is alot more comfortable then a button on the back thats set to sustain by default).
It has been amazingly stable, only once given me an error code that didnt even stop it from sending MIDI and was gone after restart. Wonderful unit!