I haven't tried the Markbass,but the Sonuus i2M I had was much worse than either the SYB5 or the Behringer clone. Latency is bad and I couldn't play anything faster than 1/8 notes, and there are additional problems with pitch variation unless your fretting is immaculately clean. Mine isn't perfect, but I'd like to think it's at least average.
I have also used a Korg G5 which was great, and the Zoom clones in the likes of the MS60B are not bad either - all of these are much more useable than an i2M.
I have the Roland VB99 which is far superior to any of the above for synth sounds, but comes with it's own problems. I haven't tried using the VB99 as a midi controller, maybe I will get round to it one day.