When I got my EF 16mm-35mm f/2.8 L lens, I also bought a UV filter for it.
The box has an amazing without filter and with filter example:
Without the filter you couldn't even tell she's wearing a hat! Besides widening the field of view, the filter really brings out her smile. And visible signs of arm folding are dramatically reduced.
All this with one filter. Amazing!
The original post had 3 comments I'm in the process of migrating over.