What makes this lens awesome is that it's an ultra wide angle lens (17mm) that zooms to become a great standard lens (24mm-40mm) for events, portraits, or street photography. That makes it really versatile, and less of a trade off than pure wide angle lenses that make it hard to shoot subjects that aren't "wide angle friendly" (like, say, people).

But here's the thing: it's a fantastic wide angle lens.
It has top-notch optics, is relatively light weight (it weighs half as much as its cousin, the 16-35mm L), is sturdy and durable, and features great ergonomics.

If you're looking to add a little creativity to your composition, a wide angle lens like this one can help you see your shot in a new way. And if you're at all interested in landscape, long-exposure night sky, or architectural/real-estate photography, then this lens is a no brainier. (If you're interested in those things, you may want to buy your own copy of this lens. Rent mine first before taking the plunge!)

NOTE: This lens is a great choice for pairing with a full-frame body (1D, 5D, 6D). If you shoot with a crop sensor body (7D, 60D, any of the Rebels, etc.), this lens isn't so much a wide angle as it is a short-distance zoom. If you're looking for an excellent wide angle for crop sensor cameras, check out the EF-S 10-18mm.
https://www.cameralends.com/rental/canon-ef-s-10-18mm-f-4-5-5-6-is-stm-san-mateo-ca-94401-usa-san-francisco-ca-usa-josh-m-4859