I've alread mentioned that I use a GR-20, and am quite happy with it, but, one of the failings of these guitar controllers be it a Roland GR type synth or a ZTar or even the YRG Midi guitar, is that they continue to constrain the guitarist to the strumming and plucking of six virtual strings paradigm. One of the main criticisms of synth technology is in regard to the dynamics or articulation control - a keyboard is not ideal for creating strumming or bowing or breath, so we create other, specialty controllers for these, and the midi guitar is just such a controller, but here again the same is true - strumming is not the same as striking a piano chord, not to mention breath and bowing techniques.
The important thing for a guitarist is a familiar noting and chording interface i.e. the neck, strings and frets; if he's got that then he can make music, and how these are articulated can be changed - new techniques of articulation can be learned quickly.
I mentioned the MISA Digital KITARA back a few posts, and I don't know whether anyone took this seriously, but the more I look into it, the more I realize that it is really the only midi guitar type instrument which truly frees the guitarist from the contraints of his instrument and will allow a new freedom of control and open new directions in creativity. There is also the Korg Kaoss Pad and the Manson M1D1 but a bit pricey.
Seriously don't just dismiss it out of hand.http://www.youtube.com/watch?v=Ek0SgwWmF9w