Just a share from someone who owns both Canon flash and YongNuo flash:
Cost aside, I think one might need to think long term. I started out with a 580EXII after consideration and advise by my commercial photographer friends - invest in the top line flash that will last you for many times to come. It's a good flash indeed, powerful, consistent, ETTL and HSS capabilities.
Slowly, I moved away from on-axis and single light to 2-lights. This time round, I bought a YN manual flash for cost savings, and also for the fact that this manual flash is used for fill light while 580EXII as key light. It need not be that powerful, and I don't mind controlling it manually.
But as I move further.. I start to realize the usefulness of ETTL - the ability to control flash power directly from my camera is something that saves me tons of time and provides me convenience. Imagine having to walk over to the flash unit to adjust the flash power here and there, or whenever you change the light direction and distance. I'm not considering a 3rd flash and my serious choice may be another 580EXII to pair with my 580EXII to place them at a distance and use the YN manual flash close to me.
In summary - if 1 flash is all you need and you don't intend to play with multiple lights or be a strobist, then maunal flashes or flashes without remote control abilities can be considered. But if you do want to play with multiple lights or flashes in future, I think a flash unit with ETTL is a better choice. I believe that YN565 is a cheap option that offers ETTL, about $200 compared to the more expensive 580EXII