In the hand, and in use, this lens very much reminds me of the old Canon 100-300mm f5.6 L: similar bulk, similar shape - but it's a version with every update imaginable: it has the very latest third generation IS (here I confess that this lens is my first so equipped, so please ignore a little honeymoon blush!) which works superbly, giving excellent results at 300mm and 1/30 second; my hunch is that it is even sharper than the peerless old L. Unlike the 100-300mm f5.6, the AF works a treat, and it is much more reassuringly built.
Rather than considering it as an update to the frankly abysmal 75-300mm IS, it might be more appropriate to think of it as a properly made 100-300mm L - but wider, faster, cheaper, IS-kitted and even optically superior - to all intents and purposes it has no CA. And it's sharper. Heck, it even focuses down to 4.9m like the new 300mm f4 IS....
This lens has 'Instant Classic' written all over it.It may not be perfect, but it really is very, very close.
* Not quite peerless: the Sigma 100-300mm f4 is sharper from 100-200 at most apertures, but the Canon is a little better from 200-300mm.