I can't speak for the MKS-50, but I have a Matrix-6, which, as has been noted, has the same guts as the Matrix-1000. It's definitely dependable; I've had to use the tuning function all of three times, and that in the first week after I bought it out of a guy's basement. Haven't touched it since. I wouldn't describe the sound as "thin," though it's definitely not as big as some of the classic OBs. It can get almost kind of delicate-sounding with the right patches, though. I'd call it a jack-of-all-trades; it does just about everything pretty well, though it doesn't really excel at much beyond buzzy polysynth brass/string type sounds. The filter is definitely self-oscillating; mine can be pushed just to the point where the self-oscillation starts to distort a little. It also has a nifty feature where the filter cutoff can be frequency-modulated by one of the oscillators, which, when combined with self-oscillation, enables it to get some almost Yamaha FM-style sounds. It's definitely capable of some good bass sounds; the low end isn't huge, but it's certainly there. As for organic pad sounds, it takes some work to set up, but the availability of two "ramp" attack-sustain envelopes, three conventional ADSRs, and two LFOs all available for a ten-connection patch matrix allows for some wonderfully complex evolving sounds. Live tweaking is limited, but it can respond to breath control and pedal control messages in addition to pitch bend, mod wheel, channel aftertouch, and sustain, all of which can be routed through the patch matrix to control whatever you like. It's not a big, impressive sonic monster, but it's a very versatile little synth, and if you need just one polysynth, you could do a lot worse.
Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73