i've once taken my car to carmax to get it appraised, and i'd say the quote was pretty fair. the experience there is pretty smooth, but i never heard of lemons being sold.
also, i think carfax's are bullshit. when i took said car to carmax for a quote, they pulled up the carfax and it was all clean...didn't show anything regarding a past accident when an H2 hummer smashed the rear...on the flip side though, the dude inspecting my car knew right away it had previous damage.. he ran his hand on the underside of the body in the trunk, and felt welds that shouldn't of been there
i rather just buy private...find one that you seem to like, get it checked out by a mechanic, look for receipts and past invoices for service, etc.
on another note, my father used this recently:
worked out really good for him, and got an unbelievable price for a new car