Look at this as a poll.
Price is a consideration.
This isn't much different than the 70-200 f/2.8 versus f/4 IS debate, except this drainpipe is $600, and the f/4 IS is considerably more.
I don't do a lot of zoom work, but the speed would certainly be nice.
I will use it more often handheld than on a tripod. I will use it for a mix of portrait/wildlife/sports, more the former than the latter. I may use a 1.4x extender.