Hi!
I've been able to use Shake and Combustion on a couple of jobs..definitely each one has its own pros and cons.
Combustion definitely gives you more features for its price point. Infinite layers, 3d camera, extensive vector paint tools, superior stabilize/tracking tools, an intuitive and easy to use particle system..and this is from version 2. Combustion 3 adds warping, time remap and a scriptable expressions tool for customized effects.
Shake gives you fantastic speed(especially on a G5), customized macros, extensive color correction tools, rotoscoping, keyer by Primatte and Keylight.
So which is better and what should you choose?
It boils down to what kind of jobs would you be involved in and the workflow the company uses.
Combustion is geared more for the individual user who wants almost all the tools found on a highend system but has no money for an Inferno. Its a toss up between using Combustion and using AE...i feel that Combustion has an easier interface but its a personal preference.
Shake is more for big post companies doing heavy 3d effects work...kinda like Weta. Since Shake offers the most in customization and speed, it allows your TDs or FX supervisors to create specialized looks and revisions for high resolution materials...believe me, i've had Combustion collapse on me using 2k or 4k layers but with Shake, it handles this with ease(sometimes anyway).
In the end, you can really co exist with either software but in some cases, would be able to complete the job only using both of them.
We haven't even included Digital Fusion in the discussion..which for me is like getting the best of Shake and Combustion...but that's another topic...
jude