I burn Buckwheat or buck rice mix in the slant grate 90,000 btu keystokers I have been burning (a top vent stove and an A90 furnace). I have heard that the direct vent units don't like buck, so hopefully you will hear from Eurpoachris on that one (or do a search on his posts). I am picking up a direct vent Keystoker 90,0000 slant grate and plan to find out exactly what the issues are with buck.
I like the buck because it has less fines and doesn't form clinkers, also seems to burn hotter when you need the extra heat. Is just a personal preference though, lots of folks burn only rice and have good results.
OK, here's the deal with buck - on a DV stove, you balance the overfire draft between the combustion air setting and the DV adjustment (damper and/or motor speed). I tried running buck and most likely didn't have my overfire draft adjusted low enough (-.02 MAX!!!!), so the stove starts sucking too much air through the hopper because buck doesn't pack together like rice and block the airflow through the stoker throat. The fire then follows the air right up the grate and into the hopper. Then the nylon parts of the Keystoker stoker mechanism do a great imitation of the China Syndrome and you're done. You are then relegated to scraping/chiseling melted nylon out of the stoker and buying some new parts. Not the end of the world, but a real panic when you smell melting stove in the middle of the night and find your hopper glowing!!!
I'd be tempted to try it again and make sure the draft was somewhere just above zero. The stove only needs enough draft to maintain negative pressure in the firebox and no more. That would probably allow buck to be run in a DV Keystoker. That said, I really liked the buckwheat - very few fines, and excellent burn with few unburnt particles. It did crunch a bit more in the stoker due to the larger size, but otherwise it fed well.