I've got one of the original non OS bigmas. I handhold it 99% of the time. Its damn heavy if you have to hold it still for 10+ minutes, but I'm better handholding than I am with a tripod.
I have been wanting to try out a 100-400 lately, but only because I want mode 2 IS for panning shots. 50% of my shots are probably pans, and it would be nice to have slower shutter speeds with the IS.
You can check out my second flickr site here:
http://www.flickr.com/photos/19426172@N08/
All of those were shot with the bigma and a 30D/5D/1D2