Hey there, I'm a Productions Manager at Flashpoint Photographic Rentals based out of Vancouver, BC. Since we don't primarily rent camera equipment, we don't have the same numbers of Canon gear as an establishment like LensRentals. That being said, we do have 20 L series lenses in our rental pool. While we've never done an official testing on our gear (though I'm considering it with the slow winter months in the shop), We've never had a single complaint regarding sharpness on any of our lenses from any of our professional clients. I myself use many of these lenses extensively for my personal shoots, and have no complaints with any specific glass.
The only problem we've ever had (in the past 3 years that I've been on board) in the shop was a brand new (out of the box) EF 24-70mm f2.8L II, and that was a mechanical issue where the zoom was jamming up around 28mm (This lens was quickly replaced with no charge to ourselves).
Hope this helps your choices! We of course are dealing with a situation where AFMA is not an option because almost all of our clients are using their own cameras. Perhaps I can give you guys a followup if I get around to doing a full test as laid out by Roger Cicala.
