I've purchased a couple dozen things from KEH over the past year or so, and the success rate is about as good as eBay. Selection is great and return policy is no-hassle (unless you buy something graded UG), but quality on average is no better than any other online seller and worse than some.
Their condition grading has become a lot less conservative of late. For example, I bought a Summaron 35/2.8 graded EX condition for $750 -- top dollar for that lens. It turned out to have focus action that was stiff in some spots, rough in others, oil on the diaphragm blades and haze in the front element group. Sure, all that could be taken care of with a $100 CLA, but for that price and that condition rating, it should have been in much better shape, IMO. I returned it, no trouble, but it was an annoyance and waste of shipping costs.
The main problem, I think, is that items are graded quickly in large volume at the warehouse, and the sales staff never sees the physical objects they're selling. There's no possiblity of asking detailed questions about the item or its condition before purchase. In the old days, you could reliably assume that a lens graded KEH BGN would be as good as something rated EX+ on eBay -- no longer, I'm afraid.
All that notwithstanding, I'm perfectly happy to buy smaller accessories, caps, hoods, etc. from KEH, which tend to work fine even if cosmetics aren't what I'd hoped.
::Ari