Its an excellent lens.
As already said, if you are going to use it at the long end most of the time,
consider a prime.
If you do decide to buy the 100-400, be aware that quite a few people have
had the image stabilizer fail (including me).
I'm 99.9% sure this is down to the push pull on the lens.
Now I'm not suggesting using the lens to zoom is the cause of the problem,
but what can happen is, when you have your lens set to 100'ish mm then
you decide to chimp, the lens slides open to its full length.
When the lens gets to its limit it tends to slam with a thump to a stop.
I believe this breaks the mounting on the IS module inside the lens
(I have no evidence to prove this).
The lens does come with a tensioning ring, so my advice to everyone who is
thinking of buying this lens, or who already has it, is to make sure you use it.
If you have a small amount of tension there, its enough to stop the lens
sliding open on its own, which should reduce the chances of it failing.
Its still a cracking lens, every camera bag should have one.
