Mike: Thanks for the link. Unfortunately, due to the language gap but also what I perceive to be a near total lack of genuine attempt at explanation or justification on the part of the author, it doesn't tell me anything useful. But if you think their carts sound great for the money, that at least is something.
Thom and Jazzdoc: Again, all well and fine impressions, but what is the comparison (if you know) to simply summing the channels of a good stereo cart that you already own? The more the folks who champion mono carts post like the question doesn't exist, the more I wonder whether they know the answer, or even to ask the question...Maybe everyone ought to state for the record (sorry!) whether or not they even have a mono switch available in their rig (or summing Y-cables, but less good for the obvious reasons), because I realize that these days many don't.
Jrtrent: I also used to take advantage of channel/groovewall-selection capability with mono records back when I was using a C-J PV8 full-function preamp, and older McIntoshes had this too...Just like a mono switch (and also an absolute polarity switch), I'd like to see modern standalone phonostages incorporate this "L to L + R"/"R to L + R" feature (since linestages won't ever again, as mono is only really important for vinyl playback). It can also be useful for certain diagnostics even if you never use it for listening to music.