When I started buying lenses for my Canon 7D (after selling my Nikon D90) I really did my research. I checked out lens sharpness, vignetting and all the standard issues that get mentioned but I could find very little info on breathing (a slight "zooming" effect while racking focus). I had read somewhere that the vintage Zeiss Jenna lenses didn't breathe that much. I also read somewhere that the Zeiss ZF / ZE lenses didn't breathe that noticeably and tested a few out - fantastic lenses. I also tested out the Canon 16-35mm and to my delight found that the was absolutely no breathing at the long end but was very disappointed in breathing and sharpness at the wide end. Apparently version II has sorted this problem somewhat (but I haven't tested the breathing on the new version so it might have changed).
The Canon 16-35mm USM II
After my research was complete and I checked my budget the choice was a few Zeiss ZE / ZFs (with an adapter) or a set of general purpose L series zooms complimented with low budget Zeiss Jenna vintage lenses from eBay. I opted for the latter as I thought this offered me more creative options and these zooms are incredibly sharp wide open - as sharp or nearly as sharp as the Zeiss Primes. True the Primes do have more available light but they're quite soft until stopped down. Check out this tool for comparisons:
Lens sharpness and vignetting comparisons
(Bear in mind that the corner sharpness is less of an issue on a cropped 7D sensor)
2 years on this is what I've learned from that decision:
Follow Focus and locked off shots
If you're going to be using a follow focus system and expect to pull focus during shots you're going to run into some challenges. In my experience with the light weight of SLRs as some of the lightweight Prosumer camcorders with detachable lenses you are likely going to have to secure your lens to a mattebox to keep your focus pulls from causing play on your lens and compromising your shots.
With a single little screw securing your lightweight camera to your rails or cheeseplate pressure on the focus gear is going to cause some play on the lens and you're going to see this in your shot when you pull focus. On wider focal lengths this might be acceptable on a pan or for handheld work but you can forget about using this setup on a locked off shot or if you're using a longer focal length. Shake or some shifting of the framing is going to be noticeable unless you secure then end of the lens to the mattebox with a threaded mount.
The implication is that lenses with moving front elements are going to be a very serious problem either for pulling focus. The Zeiss Jennas front elements move when racking focus - so these can't be secured to my mattebox and defeats the point of their purchase - the fact that they breathe very little - so much for that idea! I also got the 24-105mm Canon EF lens as a general all purpose lens which moves the front element when zooming so no zooming while attached to the mattebox.
My 70-200mm Canon L Lens and Tokina 12-24mm on the other hand are really great for zooming and racking focus during locked off shots but the 70-200mm does have quite severe breathing. The Tokina's breathing isn't all that noticeable.
The other thing that I've noticed is that breathing seems more visually acceptable when it "zooms" in the same direction as the focus pull. In other words, if you're pulling focus from foreground to background and the breathing seems to zoom in then it seems visually much more acceptable than if the breathing seems to zoom "out" when pulling focus from foreground to background.
Likewise when pulling focus from background to foreground, a "zoom out" looks intentional - as though the DOP was zooming out slightly to show more of the foreground that he has just shifted focus to. Perhaps the "opposite zoom" can be used as an artistic effect but it really makes my brain hurt - so that's just my opinion. Test it for yourself and see what you find visually acceptable.
So, to sum up, when assessing a lens for video, check the following and be aware of the implications:
1) Breathing - check that the breathing is not too severe and seems to move in the "same direction" as a focus pull (or be ok with a minimal "opposite" zoom effect)
2) Zoom and Focus - Check whether the front element with the filter thread moves during focus changes or zooming. Don't expect to be able to secure this lens with a mattebox for either zooming or racking focus
Perhaps there are some other ways of securing my righ that I haven't though of. If anyone would like to share that would be great info!