I think Blender, like all modelling packages, has a shit UI. That's just the way of things with big modelling packages. Some of the new features in the upcoming version of Blender look nice, and you can't beat its price. But I do prefer XSI's tools more.

Blender is very good.

1. It is free and open-source.
2. Its workflow is conducive to quick modelling.
3. It is feature-rich
4. It has its own integrated game engine
5. It is extremely cross-platform compatible

These are all great advantages. Blender is by no means the best 3D modeling program. But it is easily one of the best options for smaller studios and hobyists.

The biggest drawback is simply the learning curve associated with using it. The keyboard shortcuts play heavily into the workflow for Blender, and they can't be reassigned. This is a pretty major sticking point for some modelers. Modelers who prefer to customize their own key layouts tend to criticize Blender for its lack of flexibility in this area. The UI isn't really an issue. It isn't the most graphically pleasing UI. But then it doesn't really need to be, and it gets the job done just fine. The windows in Blender are extremely customizeable, its built into their very nature.

If you are at all shy about spending a lot of money on your modeling software, I would definitely recommend giving Blender a try. It is easily the best free modeler I have ever used.