The Canon EF 14mm f/2.8L USM to be exact. It is a canon L series lens so the usual standards of high build quality and sharpness along with low distortion apply. There are websites that specialise in such questions if you want that kind of detail covered.
For museum photography it looks extremely useful. Wide angle dealing with the problem of large exhibits in not so large galleries, short minimum focus distance allowing you to get right up against case glass thus reducing issues with reflections and while not the fastest lens around not exactly slow either.
So I brought one and decided to put it through its paces at the Royal Armouries Fort Nelson. It provides a good testing ground with a range of exhibit types and with lighting conditions ranging from full sun outdoors to appalling (underground in the old magazines). I was using a full frame camera so things may be different for users of crop sensored systems.
So what did I find? Firstly the lens was certainly wide angle enough to allow photos of large exhibits in small galleries. If anything the problem was that the thing was too wide angled resulting in having to stand practically on top of things to get reasonable framing. Speed wise it was fine but the low light performance of the 5DIII is so good that its not really an issue in any case. The short minimum focusing distance was pretty useful for photographing objects in the 2-6 inches range.
In terms of test shots I’ve only uploaded a couple of images but that’s more because all the useful images have already been taken:
Overall its a fairly good choice for museum photography but I suspect the Canon EF 16-35mm with a couple of extension tubes would be more useful as well as being slightly cheaper. The 14mm lens is probably better in the role of lens of last resort rather than as a workhorse lens. One area I expect it to shine would be interiors of places like the classic country house national trust property.