I used to have the 70-200 F.4 L IS lens and the 1.4 converter. I sold the converter to buy the new one Canon recently brought out, changed my mind, sold the 70-200 and bought the 70-300 L instead. I was a little worried as I wondered if the quality would be as good as the 70-200, ( because the 70-200 is an exceptional lens, and quite light to carry around). I am pleased to say that a lens specifically built with 300mm in mind is better than using the 70-200 with the extender. The quality all through is quite excellent, and this is a very versatile lens.
It is a compromise of sorts, as all zooms are. I would love to have bought the 300mm F2.8 but at £3000-5000 it's too expensive. The other options of the primes 400mm and the 100-400mm all have their charms. But I was won over by the newness of this lens. The newer IS system, the anti-smear coating, the complete weather sealing. The size ( for a zoom of this length) is very acceptable. It is a tad heavy to carry around but not too bad, but it's unobtrusive compared to some of the prime 300-400 and they are just too large and heavy to carry comfortably.
This lens can be hand held, obviously with the F stops it represents( F4 at 70mm, F5 at 135mm-200mm, F5.6 at 300mm), it is best used with a tripod in low light, but if anyone is upgrading from a standard or kit zoom, or for those who have never used the Canon L lens series, the quality is top notch. In daytime and normal light, it is excellent. the camera I use it on is a Canon 7D, I rarely use ISO above 200, but that camera can easily go to 1600 ISO without any real issues with quality. This lens works a treat.
The lens has a close focus of about 3ft, which is fine for most things, beyond that a macro lens is a must.
It has two black rings, the nearest to the camera is to focus, the furthest away the zoom. Some comment that this is unusual, but you get used to it in a matter of seconds. And isn't something that would cause any problems, unless you are a complete manual only user, which would create a new learning curve with the lens. The lens arrives with a hood and bag, but sadly no lens ring for the tripod. But these can be bought seperately.
I am very happy with this lens, it has enabled me to shoot wildlife and some action shots comfortably. It has no flare issues at all, and is extremely versatile and has great colour and contrast.
I have tried a number of L lenses, and this is one of the best I have. The others which are equally stunning( none of these have major chromatic aberration issues, or flaring issues, and work as advised on the tin) are the 135mm F2 which is stunningly sharp, the 24-70, which is heavy as a brick, but equally good. And the 70-200's and the 100mm Macro L, which is also a great lens.
So if you are like me, and need good reviews to help you make up your mind, this is a great zoom lens. The quality at 300mm ( which most will be buying it for) is excellent, and will make the expense of the purchase worthwhile. Great shots, all in one package.