Thanks for all the responses.
I did a couple of addons in gmax (but they're pretty bad so I wont release them) and I had the following three problems:
1. I found it very difficult to do up-close detail work. When I zoomed in on an object it would disappear. I think the real problem here is my lack of experience, but I point this out as something a beginner like me might run in to.
2. If I mirrored an object, it would also mirror the faces, meaning that if you have a cube with external-facing faces, when you mirror it, the faces are on the inside of the cube. The result in orbiter is an odd, "turned inside out" appearance.
3. I did a lot of my work by creating primatives and then punching out other shapes. For example, I might create a cube and push it into another shape, then subtract the cube, leaving a hole for a window in the other shape. Sometimes when gmax would recompute all the verticies, it would end up looking like a crumpled soda can. I was told on another form that I should be creating and removing faces instead of subtracting primatives, but I had problems with that technique because of point 1 above.
It's been over a year since I last played with gmax, but I have an idea for an addon so I'd like to start playing with it again. I'm wondering if I should switch to Anim8or or stick with gmax.