I traded the 50L for a 100-400L…  This lens has always been tempting for me, and I’ve realized recently shooting some sailing and surfing shots that the 300 has been limiting at times.  I find myself using it as much with a 1.4 TC (420mm f/5.6) as w/o and then at other times 300 is too long.  The 100-400 will solve those issues for me as it’s obviously a zoom.  The downside to this is that the 100-400 is only 5.6 at 300~400 so it’s not as fast and won’t provide as much background separation.  Both have IS though so that’s a plus.  Also, this lens obviously adds the ability to go from 100-299.  I’ll be testing this against my 300 as well as the 70-200 f4.  I’ll keep one of the teles (between the 300 and 100-400) and I’d possibly consider selling the 70-200 as well if the 100-400 can hang.  From what I’ve seen the zoom beats the 300 at 400 (ie the 300 with the TC) pretty easily and the prime beats the zoom at 300 (hello!).  But the zoom is enough better at 400 that I’ll take the hit at 300.  Especially to get the rest of the range.  We’ll see in testing though how the zoom performs optically.

I also don’t like the push/pull zoom style, but I’m hoping I’ll adjust.  It’s funny, my initial dislike was that it slides too easy and will end up at the long end when walking with it on a camera.  But this particular copy seems very tough to zoom with and it certainly won’t creep.  Tight is good, but it’s so tight right now that I can’t zoom with it to my eye w/o banging the viewfinder into my face….  I’ll update my feelings on this as I get used to it.