I've been running KT-120s in my M-180s for about 6 months. I'm still using the 12A*7 tubes that the amps shipped with.
Yes, I'd say it's worth it. My comments at the time I did the replacement were:
"They just give you more of everything: more treble, more colors, more drama. As happy as I was with the original sound of the Rogues, I always wanted a bit more texture and warmth on massed strings, and the KT-120s give you that."
Also, from what I've read, the KT-120 is a more robust tube than the KT-90. Depending on usage, I'd expect to have to replace 1-2 KT-90s a year (and a fuse blows when the tube blows). I haven't had a KT-120 blow yet, but only more time will tell.