I have had some experience in the past with two different varitone controls. These were put together on two different occasions by Stan Hinesley of HAS Sound, who isn't making them currently except possibly by special order, AFAIK.
This is all based on my opinion of those units, and I realize that there are a few other builders of varitones out there. Actually, anyone who is skilled with tone circuit components can put a varitone together.
Although Stan's control was well-made and high-quality, I have to say that I didn't find it that useful in a straight volume/volume/varitone configuration as the main tone shaper, at least not the choice of tones that was included. I probably liked only 2 of them, the rest were pretty nasal or out-of-phase sounding, and another is a non-filtered, straight tone.
Possibly in a more elaborate wiring harness, a varitone could be more useful, but I wouldn't recommend one as your main tone control. There's just not enough tonal variation possible. If you also added a regular passive tone control, that could be a much better combination- V/V/Varitone/T.
Also, if you could work with someone who builds varitones, and could be involved with the design process, selecting the components for the settings that sounded best to you, that would be better than buying a pre-set varitone. Still, though, that's only 5 or 6 tone choices. However, if you are a player that only needs 2 or 3 main tones for your gigs, then hey, you could go with a well-designed Varitone, vary your sounds with the pickups and your amp controls, and could have a perfect setup.
A well-thought out control harness will give a lot more tonal options than a V/V/Varitone setup. I love the flexibility, for example, of my G&L L-2500s.
Tell us about the varitone you are thinking about using. Who built it/is building it for you if you go that route?