I do, indeed, own a CK20HST that I purchased in Jan. 04, nearly 350 hours trouble free so far.
I do know that some owners have had issues with the MMM, i.e. on the side discharge, too much material coming from the front and clogging the radiator too quickly, and on the rear discharge, the pully being rather small and having problems keeping the belt on.
Also, pinning the mower up when not in use has sometimes been a problem for some.
Now, all of that may have been corrected by this time, but those issues have come up, and my dealer had to send some back due to those problems.
As for the tractor, depending on your needs, you may wish to go up to the CK25 for a lot of extras that the CK20 doesn't have. I've been on nearly every Kioti model, and have, several times, been on the CK25 and find it to be very strong and useable. Some say (only non-owners)it is underpowered for the frame size, but I don't agree.
I do an awful lot with my CK20, but if I were buying today, I'd go with either the CK25 or 30 for the extra power and features.
John