There has been lots of news in the area of MIDI controllers in 2013. This video, though, demonstrates that you can a lot with just a guitar, ThumbJam & SampleTank.
I am not discouraging anyone from buying any apps here. Just showing how I use Thumbjam to achieve sounds close to what the dedicated midi conversion apps do.
For anyone who is serious about this kind of feature, I would recommend checking out the Midi Guitar app, as it is certainly tracks well and is polyphonic.
One last thing I’d say is that you have to practice with these apps. You can not just play your guitar like you normally do. Notes have to be muted more, there is a bit more latency, sustain will confuse it, vibrato jumps around. It takes time, but hopefully I demonstrated that you can get some very playable results.
Check it out and let us know what you think!