It's portability versus compatibility really, isn't it? Direct 3D will run far more flawlessly than OpenGL on a higher percentage of current hardware, unless you resort to various ugly hacks (witness Doom 3's multiple render paths), which kind of takes away from the fact that OpenGL is supposed to be easier to use. You get your basic code up and running fairly quickly, but once you start pushing the envelope you end up having to put in an equal amount of work to get there. The really nice thing about Direct 3D (especially 9, I don't know 10, and what I've seen of earlier versions makes me wince) is that it really "just works". Once you get over the whole Vertex Buffer and Lock/Unlock thing, it's actually a very nice API to code for, if a little on the verbose side at times.
Anyway, I think I have an almost complete port now; just a few days of tidying up and restructuring to do, and I'll release it.
Oh, and .
