I've had one for about a month and love it. It is really sharp, really really quick to focus, nice Bokeh, good in low light or at least meets my early morning/evening needs, smaller filter size so cheaper. It is a little heavy but all L lenses are, I have carried it round a Zoo all day on a 500D without complaint but have just got a BlackRapid sling which helps no end, now I take it out with me on nearly every dog walk.
I got it for general outdoor stuff and to get pictures of my dogs and others coursing which I am in no doubt at all that it will be up for the job and looking forward to the next meet.
I tried the 100-400L which is heavier, older and strange zoom imo.
70-200L 2.8 was highly recommended but heavier, longer and 100mm shorter without a xconverter and with xconv its a £1000 more and even bigger.
70-200L 4 IS was ok but 100mm shorter and not as nice to hold, just did not click with me.
There was nothing else that even came close to it in terms of image quality, speed or build.
The 70-300mmL ticked every box for me (apart from price) for outdoor photography and everything about it compared to my Tamron 18-270mm is unbalivable and worth the 3x more easily.
I am saving for a 7D (and 15-85mm) which is supposed to really bring the very best out of this already awsome lens.
Oh the IS is fantastic and 2 stage as well so good for panning, I took this hand held
HAND HELD 1/2 sec f45 @300mm as a test.
Instantly focused on this
And this through a fence
by
PINNACLE PHOTO LOG, on Flickr
There is a Flickr group if you want to see more.
http://www.flickr.com/groups/ef_70-300mm_f4-56l_is_usm/
I cannot fault this lens in any way and after owning it now for a while I would go as far and say it is worth every penny. If you need 400mm the Kenko x1.4 works with it and you keep auto focus (albeit slower) I have been told. The tripod ring can be found on the net for $28 I think I remember seeing. Canon want £200 or something stupid but coming down seen one for £164.
Amazon were the cheapest when I shopped around and they had stock where as a lot of shops I called didn't.