well expensive stuff is Sonus (though I think it's super basic, which might as well be acid i think was the message during the discussion)
I personally use P21S Wheel gel, and I mean, the only thing that I've had trouble with was my BMW but that's because it needs to be refinished--it's definitely made my life so much easier...
I am however looking for a cheaper product, as I'd rather save my p21s stash for higher end clients or situations that require some more caution....
as far as I know Purple is rather harsh to the environment and such, so I steer away from it (but I hear it's pretty powerful stuff for engine bay and undercarriages too)