I've used Combustion, but pretty much everything I did in it I can now do with Shake and Motion. It had some nice things with particles that I still keep asking for in Motion - like Particle Reflectors, but overall I'd say I'm more productive with these tools than I was with Combustion...
If you take a look at my
Visual Effects reel (click on the Visual Effects button), two effects were done in Combustion. The bluish shot of sad people, and the shot of the radiator boiling over. All the rest were done in FCP, Motion, and Shake...
Patrick
P.S. I'd also venture to say that AutoDesk wasn't losing money to their lower end product range, rather they feared they would, so they crippled them and made sure that Combustion wouldn't be competition for Flame/Smoke/etc...