I am rating this lens a 10 for what it is. I am not comparing it with other lenses because there is absolutely no lens on the market that is similar to this one: a constant f/2.8 zoom to 300mm.
I've had this lens for over a year now and I've got brilliant results with it. It's very sharp even wide open and I don't hesitate to use it at f/2.8 because I know it produces brilliant results, and that's quite amazing for a zoom lens. It does get better at f/3.2, and at f/4 it looks as if it went through serious USM in PS.
It handles purple fringing quite well, and it's only noticable wide open at extreme situations, which is easily fixed in PP.
300mm is a bit short for birding, but it does work great with both TCs. The 2x TC had to be stepped down quite a bit to get the sharpness I was after (I am a pixel peeper to be honest) so I tend to just use the 1.4x with this lens.
The lens hood is a bit small for this lens, compared with Canon's 300mm 2.8. I do get the occasional flare when I aim towards the sun, and I think that with a bigger lens hood this could've been avoided.
Some deal the lack of IS/VR/OS as a negative. Why? I know it would've been a better lens with stabilisation but people know this before they buy the lens. If it had stabilisation and it was rubbish, then yeah, that would be a negative aspect of the lens. But this is a non-stabilised lens and we should treat it as that.
I find the size and weight of it quite convenient. I hand hold it all of the time, and I only use a tripod for when I am in a hide. It's really manageable hand-held, even though there is a learning curve for it. The weight for it helps with vibrations and if you use proper long-lens techniques you'll manage to get sharp results even with slower shutterspeeds. I manage to get tack-sharp photos at 1/150 sec. regularly. At that shutterspeeds I tend to take 3-4 photos to make sure that at least one is sharp, but I never fail to get a sharp photos at that speeds, and I think that it's down to the lens' weight.
The matte paint is bad. It does make it feel a bit warmer during winter, it doesn't slip either. It does what it's supposed to do, but it's easy to scratch, and after a year it's starting to peel off. I have been treating it quite badly to be honest, but it shouldn't be that bad. My advice would be to get a lenscoat for the lens as soon as you buy it, if you want to keep it in mint condition. I don't really care about any blemishes or peeled paint as long as the lens works, but most people do.
All in all a great lens. I am in the process of buying a long prime, probably the 600mm, and I am doing my best to avoid selling this one because I really love it. My excuse for keeping it is that it's a great focal range for larger wildlife, and it's an amazing lens for BIF shots!
