I fitted one to my E-Tuned 36 Rhythm on a Spectral on that was not using the full travel.
I'm light at 65KG fully togged up.
It made a massive difference to the sensitivity of the fork and the travel used. Very plush and it allows you to hammer through rock gardens. The Rhythm fork gets a bit out of its depth in really rocky terrain but the TruTune Insert stops the worst hits hammering your body.
However, I also fitted one to the Fox factory fork on my StumpJumper Pro from new.
It completely messed up the damping and was harsh as hell. I messed about with the air, comp and rebound settings trying to find a good set up. All to no avail.
It was only when I removed the Trutune Insert that the fork finally started to work like a grip2 damper should. Plush and controlled over all types of terrain.
So maybe it is more suitable to a damper like the Grip. Nothing wrong with the basic grip though and in a lot of situations, I think it is better than the grip2 or X.
Mainly because you can quickly change the compression damping to suit the terrain with a quick turn of the dial.