I've tried the lot of opensource 3d apps including Wings3D, Blender, Aztec3D and Anim8or.
Of all, Wings is probably the most limited IMHO.
Blender has the steepest learning curve but is definitely the most mature.
With that said, most 3D artists use a range of tools instead of just one.
Get them all and find a workflow which works for you. After all, the're free.
Also try SharpConstruct. I never got it to work myself but I'm still very excited about this one.
Really, I haven't had a good experience at all trying to get stuff into Nexuiz. The formats are simply unsupported unless you go for 3D max et al. You will have to go through milkshape or some other converter to get it into a format to convert it into a Nexuiz model.
If someone could do some documentation on the converters...
I think that this has been the bottleneck in the development cycle from day 1.
I have spoken face-to-face with a couple 3D artists and the reactions were the same:
1. Work for free? Why?
2. ZYM? WTF!
It's SDK time IMHO.
P.S.
There is a PERFECTLY WORKING MD3 import/export for Blender which I found poking around the xreal site
http://xreal.sourceforge.net. It's the one I used to import the MD3 weapons to the native Blender format. Can't find the link right now but I can always host it somewhere. I guess it's GPL. They don't specify.
These XreaL guys seem real serious about making a next-gen game out of Quake 3. I honestly think they are Nex's only real competition.