Zig pretty much nailed it with that post. I'd say for 90% of the DSLR users here legacy glass would be a non-starter. It appeals to a certain type of shooter. It's not easy, takes patience and you have to be willing to miss shots.
BTW Zig, by fast 400mm i was thinking more along the lines of the Nikon 400/3.5 which would actually be about the same as a used 50-200 with ec-20 but would be faster. Sometimes the extra speed makes all the difference. Here's a comparison of shots taken with the Sigma 135-400 on the left and the Tamron 80-200/2.8 on the right. Note that despite being taken at 800 instead of 1600 I could still get a faster shutter speed and a better shot with the Tamron.