I had them in the 11A and they were never an issue. I did eventually get a hold of c-j customer service and was told it was okay to use them. Although they run at a higher heater current (1.8 amps) than most current production tubes, the 11A was designed and voiced around the GE 6550A which runs at the same heater current. Vintage Tung-Sol 6550s also run at 1.8 amps and I'd previously had both the T-S 6550s and the GE 6550As in the unit without any issues.
conrad-johnson is now supplying all of their current production amps with the KT120 and both Lew Johnson and Bill Conrad have both been quoted as really liking this tube. That opinion seems to be shared by most c-j owners, too.
I like the tube, but it seems I'm in a small minority of users, maybe just me, actually :) who prefer other tubes in my c-j amps. The New Sensor Tung-Sol 6550 and New Sensor Gold Lion KT88 being two current production output tubes that I prefer.
The KT120 is smooth sounding with excellent imaging. It has a solid bass response and the treble is generally also good. However, I find it a little lacking at the very top and in my opinion, it also lacks a little of the upper midrange, lower treble clarity of a good 6550.
Just my opinion naturally and I want to restate that I seem to be in the minority.