Honestly, it would take far less time and effort to start from scratch and build your own model...
I've never played with any decompilers.. I've got my hands full using that mental energy getting the most out of a model MAKING program.
Fr. Bill is correct.. by the time you'd get some sort of useable model out of it.. you're still looking at sorting through it.. re-animating it all.. and mapping textures to it. I have a hard enough time mapping ALL the parts of a model I've made myself. I can't imagine trying to make sense out of someone else's work.. It would be a larger task than just learning how to make the model yourself.. from scratch.
That said.. I'll admit to having used other peoples models for reference and ideas. FreeFlightDesign has had Gmax source files available for years.. I've never "taken" parts from those.. but I have certainly experimented with them.. So I suppose a decompiler would be good in that regard.. But again.. you get into the gray-area where it's probably easier to just learn it yourself..