Deep Exploration is the cream of the crop for this kind of stuff. I bought a 3 year license with them because I ended up relying on it for work at the time, clients often wanted models in various formats and it was easy to just convert them with this.
If it's not out of your price range I would highly recommend it. It's not really just about the amount of formats it supports, it's mainly to do with that I never had any problems with any conversions. Models went in and out exactly the same, which is a rare with a lot of model converters.
During my c++/directX fiddling I noticed that it came included with a command line based .x converter, I never used it so I don't know what the deal is with it but it might be worth checking it out.
Also never used
quick3D and it doesn't mention anything about batch conversion but it can't hurt to give it a try, it's also a hell of a lot cheaper than Deep Exploration.
I'm also sure that with the right libraries it wouldn't be too hard to create a quick batch conversion program, maybe someone on here has already done something already?
