(Note: Lenses are listed in order of starting focal length)
This is really one of the lower priority lenses you should consider if you are just getting started. I would say the Canon 16-35mm 2.8 L II is a more important lens to get first, but the 8-15mm Fisheye Zoom is a tool every serious pro will eventually want to have. It is great for Extreme Wide Angle Shots, Landscapes, Event Shooting, Water & Abstract Photography. This is my favorite lens when doing surface water photography. Event shooters will love the zoom feature, Works excellent on both 1.6x and full frame cameras.
I was on the fence about this Fisheye Zoom for a couple months before I actually went for it, and I have not regretted it once. The more I shoot with it, the more I discover how useful it really is.
Im not a fan of having to have the lens hood on in order to use the cap (I usually get rid of my lens hoods as soon as I get them). Also not a fan of the curved glass but no way around this as it is a fisheye, just be really careful not to scratch it.