Sigma Ring Flash vs. Canon?

I'm planning to buy the Sigma Ring Flash for macro work very soon, but I was wondering, why is it so much cheaper than the Canon equivalent? I can't afford the Canon alternative at the moment, but is the Sigma flash suitable?

I can't seem to find any decent reviews and I've found no comparisons, so I was wondering if anyone could help me out?

Also, do I need to buy one 'to fit Canon'? On some sites I've found it doesn't list the brand that it fits, and on others it says either 'for Canon' or 'for Nikon'.

Either way, I'm planning to buy it from the following website:

http://www.crazycamera​s.co.uk …roductId=9202&a​ff_code=gg

Would this be okay?

Sorry for all the questions but I just want to ensure that I'm making the right decision!

I thought I had already answered these questions for you?

Yes, the flash works fine on Canon cameras. You need to determine which lenses you'll use with it because you need adapter rings to fit onto the filter threads of the lenses. It comes with adapters for 55mm and 58mm filter threads. Make sure you get the flash for Canon. It's found here. The one you listed is for the Sigma SD-14 camera and will not work on Canons.