I used to have one, and honestly wish I still had it for the price I got
It's a great lens, but I just don't use that focal length range on walkabout and a 100mm macro plus 1.4x telecon won that particular space in my bag. PX'ed it for a 100-400mm L which is great as you will know as you've got one, but as similar as these two lenses are in many ways, they couldn't be more different to use.
Don't pay too much heed to reviews that say the 70-300mm DO flares all the time, or that it sometimes has strange bokeh. I never noticed either after a couple of thousand pics, including a Greek island holiday where the sun was permanently mega-bright
Use the lens hood supplied (I always use a lens hood anyway) and you will be rewarded with clear, sparkling images, fast focusing and good IS. People testing this lens know it is a unique design that has some optical querks and so, if you go looking for them, you will find them and post stuff about it.
Optically it is on a par with the regular 70-300mm IS, which is to say pretty damn good. Where it really scores is on physical length, when packed away in the bag for carrying. It's amazingly short, but it is also fat around the middle, and heavy. It extends a lot at 300mm and is certainly not unobtrusive, especially with the hood on. I also found the zoom ring a bit jerky but what annoyed me most was zoom creep - hang it down on the camera at short focal length and it will zoom out to 300mm immediately. I've heard some people even use the zoom push-pull style because of this characteristic! The lock only works at 70mm.
This len's ace card is compactness. That's it really. It will sit upright in your Slingshot bag no probs, which maybe your 100-400mm won't
If this means you take it with you rather than leave it at home, then it's done its job.
Regards,
Richard.
