I've been using .x mainly. I've had trouble with .dbo files (which may say more about my inexperience than the format).
I've found that an object built in DBP and exported to .dbo will represent the geometry incorrectly when the file is loaded again.
I've found that the programs which export .dbo format (Fragmotion and UV Unwrap) will only export animations in ticks, rather than keyframes, which makes timing an animation developed using keyframes difficult in-game.
I've also had more problems trying to apply bump mapping to .dbo files than to the .x equivalent of the same model.
The only advantage I've noted for .dbo so far is that the files are smaller than text .x files, but I haven't compared file sizes with binary .x.
The .x format is also better documented. As far as I can tell, there's no way to open up a .dbo file and look at the internals, which can be useful if your 3d object has some problem which you need to track down.
Maybe someone will come along and explain the benefits of .dbo to both of us.
I don't mean to sound like I'm slamming the format. I probably don't know how to implement it properly.
