I've been working with Combustion for five years and it really filled all my needs. But now that Autodesk is about to release Combustion 2008, the future of this application is becoming unclear.
I've seen various compo applications that surpass Combustion capabilities to make the switch (Motion itself, AE, Fusion...) so i'd be glad to receive some feedback of Motion users, or better, Combustion users that had switched to Motion.
The main need we have is the render farm. Now we have backburner software with 6 computers rendering sequences of frames. That allows me a big volume of work in a day. Is Motion capable of that?
We have Sapphire and Trapcode plugins. I don't know if they're compatible with motion or if Motion has something equivalent.
Has the 3d environment of motion light and shadowing effects? Is it a real 3d or a flat simulated 3d like Combustion?
Motion can do many things that are difficult or impossible in Combustion, but on the specific features you asked about:
Motion uses the GPU for almost all rendering, so it doesn't support distributed rendering like Combustion and AE.
Red Giant now sells FxPlug (the Apple filter API) versions of most of Trapcode's plugins. The Sapphire plugins have not been ported to FxPlug, so they can only run in Motion on PPC machines, where Motion still supports AE plugins. There are a decent number of FxPlug filters out there, so there may be equivalents for what you need from Sapphire.
Motion has point, ambient, directional and spot lights, but no shadows or visible/volumetric light sources.
Motion has a full 3D environment, but all the surfaces are planes. Text, particles and replicators can have each character or cell move independently in 3D, however, so you can create volumetric effects.