KMX had a sort of herbal, medicinal taste (sort of like Materva, which I like)
Rockstar, on the other hand, tasted acidy, like drinking unsweetened powdered Kool-Aid. (It does come in that huge can, though....and did seem to do the job of keeping one awake).
Unfortunately, KMX is on the way out and more than likely Coke will try to replace it with one of its European energy drinks, like Burn or Play.
Rockstar seems to be doing pretty well, especially in the West Coast. They seem to be establishing themselves as the solid #2 in the energy drink category. Time will tell if their 8.3 oz will help them occupy more market share.