The Tormek stuff is really good. It's 0.5 um Al-Oxide IIRC, and appears to be very tightly graded, yielding results similar to what I get from tightly-spec'ed 0.5 um diamond pastes. I also have some 0.5 um Al-Oxide powder that I bought from an optics house a while back, and that works nicely.
As has been pointed out many times, the green stuff isn't actually 0.5 um as claimed, but IMO it's sufficient (for some definition thereof :-) for woodworking. I believe that George uses it, for example. I wouldn't use it after any stone finer than about 1.2 um (8000# on JIS scale) though, as it only makes the edge more jagged in my experience.