My 5 cents on MPE. I have been eyeing with interest the all-new-and-shiny keyboard tech for a few years now, and have always resisted the occasionally passing idea of investing some significant money in a Roli or Osmose controller. This is partly due to the fact that I do not particularly fancy mid-size keybeds for my playing needs, and the 5-octave Roli Grand sports a price tag that for me would rather call for an A-class synth acquisition (like, say, a Moog Muse).
This being said, a technology capable of rolling 15 Midi channels to address polyphonically volume, pitch and timbre together on a per-note basis is simply amazing per se, and certainly affords levels of melodic and chordal expressiveness never attained before by electronic musical instruments.
I quote from the Roli site MPE explainer:
[When a device is transmitting MIDI according to MPE, it will use a range of channels:
One channel (usually Channel 1) is used for global messages – data such as preset changes and pedal positions are transmitted on this channel to apply to all notes equally. The global channel is set by the “MPE Zone.” When the MPE Zone is the Lower Zone, Channel 1 is used for global messages. When the MPE Zone is Upper Zone is, Channel 16 is used for global messages.
The remaining channels (usually Channels 2–16) are used to transmit notes and expressive data – including note-on velocity, pitch bend, channel pressure (aftertouch), CC74 (brightness), and note-off (release) velocity – on a note-per-channel basis. The expressive data will only apply to the note which is on the same channel.]
As each new technological breakthrough, MPE is not necessarily an enlightening revelation for every musician on the same level. The only time I ever laid my hands on a Seaboard I received mixed impressions: it was indeed extremely expressive and flexible - maybe in the way guitarists, oboists or string players are accustomed to - but not necessarily musical to the same degree for every genre of music and musician.
I guess that for someone doing Raga, ethnic, blues, jazz or improvisational stuff, or maybe mounting a keyboard orchestra for contemporary classical music, MPE controllers are arguably the new way to go, since they allow local note control at an unprecedented level of detail and natural feel. But IMHO this depends a lot on the kind of musical culture and instrumental technique one adheres to and/or aims at. My musical world is polyphonic at the core, and my focus of interest is structural development of thematic ideas, therefore I would seldom resort to the luxury of a costly 3-D Midi controller, for some particularly striking solo passage or theme exposition, let alone the fact that managing MPE data in a DAW for ensemble/orchestral programming is challenging and time-consuming.
Furthermore, there are anatomic limitations, to the way a human hand can actually convey independent musical lines with specific pitch control - like say, a gamba player - and the real number of truly autonomous melodies that a keyboard performer can sustain is only three or four, with some compromises and quite a bit of harmonic filling notes. Thus, for how appealing would be to (try and) perform the keyboard rendition of a six-part Byrd’s song for viol consort on Equator, with an MPE controller, the unsurmountable reality of the human limbs is that meaningful pitch and timbre control per-finger is intrinsically outside the instrumental nature of a keyboard, divided in discrete steps even if it is clad in sensory rubber, and mastering a polyphonic 3D concertato writing is probably attainable (with much exercise) for no more than two or (perhaps) three voices.
Anyway, I would love to find the time, resources and opportunity for exploring the possibilities of MPE nuanced playing, but for the time being I am still quite content with stepped tuning, devoting my musical energies to the composition of larger canvases (BTW, and however most of us seldom/never use them, there exist interesting and sometimes revelatory tuning systems alternative to the equal-tempered, like Mesotonic, just and Pytagorean, which already project the performance and its tonal character in unusual territories).
Best regards 
