I dont know if Tangent works with FCP 6, but My JL Cooper Spectrum does. You need to contact JL Cooper to get the beta drivers.
The rings work really well for low mids and high, but the balls can be a little sticky, but they do work. As a matter of fact, I recently was playing with FCP's CC with the SPectrum, and got a better look than I would have gotten with the mouse alone. I'm not not sure exactly why - I'm very experienced with the 3 way CC, and use the scroll wheel to adjust gamma, etc, but somehow the Spectrum let me dial in a nice correction.
ANother plus, I'm not sure if intended, but in the motion of clip- the middle trackball
controlls position - great for panning stills. I just wish I could figure out how to map the percentage of zoom size to a one the Spectrums rotaty encoders.
Saturation comes up in the rotary encoders of the Spectrum, which is nice.
The Spectrum is really killer with Color, and the MCS3000/3800 works well with FCP, so all in all, if you have the money, tactile controllers are the way to go. The Tangent is just too expensive in my book, so I'm glad I never tried one and don't know what I'm missing. For the $20,000 + is costs, I mean hell, you could get a really nice Turntable, Tonearm and cartridge. I'll stick with the Spectrum.
And By the way, the new JL Cooper Eclipse, while it looks really cool is just a Spectrum and MCS 3000 in a fancy housing, for quite a bit more than the cost of both the plainer controllers.