My only interest in Blender is animation, rendering and python scripting. I use Silo and 3DCoat for modelling, so I am not interested in a full 3D package.
Blender might not be the very best for rendering, but it can do the job. Animation it is at least superior to LW whilst it has had Python scripting for years. The modelling and sculpting tools are merely a bonus.
I just don’t see the point in shelling out money if Blender can do those three jobs - well enough - for free. Might as well put that money towards modelling tools(where I struggle) and new and better hardware.
Saying that, if Blender didn’t exist or wasn’t free I would go with Lightwave instead…
Edit: Sorry, forgot to answer other parts of your question. Yes I have used other software, including Real3D, TrueSpace, C4D, Lightwave, Maya and 3dsMax. Also, being Open Source doesn’t matter that much to me, but I think its a good thing as it allows Blender to grow fast. In previous years Blender has had to play catch up, but now it is in a position to stand proud alongside the other 3D packages.