Unlike most slim/wide-angle filters, this Sigma includes a front thread, which means you don't need a special push-on lens cap; the one included with your lens will work just fine. That's a major boost in convenience.
It also means you can stack additional filters, though beware of vignetting.
A very thin polarizer that works well and does not vignette. The only negative is that it came with a big finger print in the middle of the outer glass.
[3 of 3 customers found this review helpful]
I bought this to go with my Sigma 10-20 UWA and Canon 17-40L. It does a fabulous job on both. Its thin-ring design means no vignetting on ultra-wide angle lenses. It stays on my 10-20 (until I put my 17-40L on!).