Here's why: It's small, light, cheap, fun to look at and.....it delivers wonderful and pristine optical performance even when used at its widest aperture. It has a wonderful combination of bite and realism. It's capable of high resolution with high contrast and, one of my favorite lens test sites, Lenstip.com, raves about the overall imaging performance of the lens -- across all the systems for which it is available.
For people who need wide angle lenses because they just can't make up their minds about what needs to be included in their photos and what needs to be excluded from their photos it might be a little long (focal length, not physical dimension...). But, if you like to isolate subjects and have definitive ideas about cropping the focal length is very good on the m4:3 cameras (equivalent to a 120mm on a 35mm, full frame camera) and perfect on an APS-C camera (90mm equivalent).
It also has the added benefit of being able to focus quite closely as you can see below. While it won't take the place of a good macro lens it will let you cut out a lot of extraneous clutter while maintaining high sharpness. My impression about the quality of its out of focus areas is that they are some and smooth and very desirable. (See image just below).
The lens comes with a hood and a small case and is around $240. While the f2.8 aperture may seem inadequate for some low light uses the two advantages bestowed by the limited f-stop are: It's fully sharp and usable wide open and it's comfortably small and portable.
I consider it one of the great bargains available for all of the cropped sensor systems. You should rush out and buy one right away. If it doesn't fit on your full frame Canon or Nikon DSLR camera (it doesn't) then this gives you the perfect excuse to finally get rid of that old clunker and step into the wonderful world of smaller and more capable mirror-free camera systems from Sony (APS-C), Olympus and Panasonic.
I figure that any lens worth owning three times over is a lens you'll probably want to try.