I use a UC4 with my octatrack, but not ON my octatrack. It's actually used to control the Nord G2 Engine that sits below. BUT, I can say that using it with its MIDI hard out (no computer) is totally painless. I was nervous because it doesn't have a software editor, but it's shockingly easy to program from the front panel. Also, with 8-pages for the faders/buttons, and 8 pages for the encloders, which are switched separately, you can get wherever you need to. It's pretty impressive. I have only the best stuff to say about the UC4.
Personally, the only MIDI control I really want over my OT is to bring out per track mute and cue buttons. I find the crossfader and an occasional direct encoder tweak cover anything else I need to do. But different strokes for different folks. If you want per track volume and pan or whatever, UC4 would be great for that.