I still have a Yamaha KX5 strap-on, and still use it extensively for jamming out or sitting in. I use a Roland MGS-64 module with it. It is the perfect tool for sitting in without taking up hardly any space, and I love the 'one hand' approach for jamming, stops you from playing too much! I even had a 4-piece gig at a casino in Tunica that the stage was SO small ('how small was it..?') that there was NO room for a full keyboard, and I used just the KX5 for about six months! You learn a LOT about 'less is more' that way!
Other, more recent strap-ons have far better MIDI implementation, and bigger and more keys, but there is something about the KX5's pitch strip that I find MUCH more comfortable to play. It just falls under your fingers like a good guitar neck, and allows you to do incredible things like hammer-on/offs, trills, legato passages with horn sounds (just 'jump' to the note instead of playing it, and the note doesn't re-trigger - kind of like a poor man's SA voice), pedal steel licks.... I love it!
But it's MIDI sucks! Only PC#'s up to 64 (that's why I use the MGS-64, you can reprogram the PC#'s), only velocity to about 110 (like the old DX7), only MIDI channels 1&2... It's definitely a relic! I wish Yamaha would re-introduce one with modern MIDI but the same form factor (especially the controls)....
I have touch strips on my K2500 and Triton, but there is something different about using one when your hand wraps around it like a guitar neck. It just feels natural, and I still tend to use the KX5 for studio productions to drive other synths when I need touch-strip bending tricks.
Anybody else still using strap-ons? (No, not THAT kind, Donny!
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