The Canon TS-E 17mm f/4 L is an amazing lens although beginners will struggle to master its possibilities at once. When using the lens based on conventional settings it behaves pretty much like a normal ultra-wide prime lens. The center performance is pretty much excellent followed by good borders/corners at f/4. At medium aperture settings it's very sharp across the frame. The level of barrel distortions is unusually low. Same goes for the vignetting figures as well as for lateral CAs. The lens is somewhat more prone to flare due to its bulb-like front element.
The real power of the lens is unleashed when using the tilt-shift features and their corresponding capabilities in terms of perspective- and depth-of-field control. The Canon lens is like no other here - it's the only ultra-wide tilt-shift lens for conventional SLRs. The shift aspect (perspective control) doesn't come for free though. Depending on the amount of shift it increases the vignetting (single-sided) and there's a penalty in terms of resolution. Typical for tilt-shift lenses you've to live without AF and the conventional light metering tends to be off when shifting an image (Live-View behaves fine here). The build quality is absolutely stellar but then you can expect no less from a lens in this price class.
Technically there may be lenses out there which can match and perhaps even surpass the lens for conventional photography but the Canon has a creative potential which you will not find anywhere else. Consequently it's ... highly recommended!