The simple answer for me is that the Kohler Rialto toilets came in colors, and the colors are very difficult to match with the existing tub, sinks, and tile. So I keep nursing them along until I do a bathroom re-do. Could pretty easily plop in a white toilet, but it wouldn't look right (we have one grey, one pink, one raspberry (sort of a dark red, burgundy-purple). Had a talk with the local Kohler dealer, did not seem we could even custom order one. Lesson learned, from here on in buy white.
I recently stumbled across this thread again, as I was having trouble with one of the Rialtos that I rebuilt with a Cesco Brass Ballcock:
http://www.cescobrass.com/Model16ak.cfm
and a Kohler Flush Valve kit:
http://www.us.kohler.com/us/catalog/partProductDetails.jsp?partNumber=83650
The water was running, wasn't shutting off. Poked around, and it appears that the Kohler Flush Valve washer is disintegrating, it's a $3 part:
http://www.us.kohler.com/us/catalog/partProductDetails.jsp?partNumber=53410
so I'll probably replace it shortly, but had other projects, so for now tightened down on the screws for the lugs (I could tell there was some problems because one was loose to essentially finger touch), seems to be OK for now. Would have hoped for a little more life out of the Kohler replacement part, but oh well.
I'm pretty happy with the Cesco Brass ballcock as a replacement for the Kohler part, the diverter valve was a bit tricky so I just stuck with the old Kohler one, and the unit has performed fine, at least so far. It is a bit slower/quieter than the Kohler replacement part (which I have in another of the toilets).