The SynthAxe was one of the early guitar MIDI controllers, built about 1985. In addition to a guitar-like fretboard, it also had a few keyboard-like keys and a pedalboard (which also held the power supplies).
Unlike modern MIDI guitars which attempt to detect the frequency of each string and convert it to MIDI data, the SynthAxe used a different approach. The strings of the SynthAxe don't create sound on their own, but are merely sensors which convert the guitarist's finger positions to MIDI data. It does not need to use different size strings for each of the six strings, so easily-stretched strings are typically substituted in all six positions. Both fret position and string bending are sensed, so the player can perform natural guitar pitch bends. Notes can be triggered from the second set of strings, strummed or picked like a guitar, or from the keyboard which was velocity sensitive and had polyphonic aftertouch. It also had a whammy bar which could be assigned to any MIDI function desired.
Users say that the whole assembly is so heavy that it's more comfortable to play sitting down than standing with the weight around the player's neck. Also, the non-standard spacing of the frets take some getting used to. However, there is no delay as there is in MIDI guitars that track string pitch ... bends, hammers, pull-offs and trills all respond instantly. However, these devices were very expensive and were only made for a few years before the manufacturer folded.
Famous musicians who used one, for a while at least, included Allan Holdsworth, Al Di Meola and Lee Ritenour.