Yep...I first encountered the variac on my Roadster and fell in love with what it did for the tone & feel. Now I'm addicted to it. My next Mesa must have one!
I always feel it as making the amp more playable and elastic feeling, and hear it as making the overall sound slightly more scooped with a little bit of the extreme high end rolled off.
I find there's a bit more harmonic swirl in variac. More compressed. I agree there's a bit less top end but there's also a bit less bottom. I started using it a year ago and use it about 90% of the time.
There's a point of diminishing returns on the master volume. Once the amp is being pushed fairly hard it turns into a pile of mush and switching to full power may be necessary to restore articulation.
It takes some getting used to and I'd recommend not switching back and forth between variac and full power in an effort to compare. Take the time to get a feel for variac, adjust the EQ if necessary, then after a few days switch back to full power and compare.
I too use the variac during rehearsal. When I gig indoors, I usually leave it on variac 90w. But halfway into a gig, especially outdoors when the rest of the band or crowd is feeling it and everyone else feels the need to get louder, I flip it to full power and open it up!