Well I've got the 500mm f4is and the 100-400 so here comes a biased opinion.
I think this setup works well, I've used the zoom for all sorts of photography at everything from 100mm - 560mm and have been more than happy with the results. My shots with the tc on aren't a patch on Keith's but I still got decent shots from long birding walks I wouldn't otherwise have got.
The af isn't lightning fast, but with the range limit set its fast enough virtually all the time, and should be noticeably quicker than your old Tamron.
IQ, no complaints at all
Using a tc on a 40d could be a problem. The pin taping trick seems a bit hit and miss. There are other threads going into detail on this, so I'd have a good read of them.
I did consider the 300 f4 but felt the flexibility of the zoom outweighed the possible benefits of a 300mm prime and nothing has altered that opinion in the year I've had the zoom
Paul
Edit: I've just knocked together a quick gallery of shots with the 100-400, so if you're really bored go here
http://www.pbase.com/pauls_20d/canon_100-400is_sample_images
Edit II: Should have said, all shots handheld, first thing I did with mine was to take the tripod mount off and chuck it in the back of a drawer. Its a doddle to handhold!