Flash - other make question

I don't use flash very often, but my question is - is it safe to use a non-Canon flash unit on a Canon DSLR (I have 500D and 5D at present)?
The Canon 580 for example seems a lot dearer than some of the third party makes; however I would go for Canon if there could be a problem with other makes' units compatibility.
I would welcome any feedback/comments on this.

You want a flash that is ettl compatible. Thats the auto flash control system canon uses. There are other brands that make ettl compatible flashes, sunpak, sigma, and metz come to mind. I would consider the canon 430ex or 430exII, a stop less powerful than the 580 but still a very good flash. What you want to avoid for use on your camera are older flashes with high sync voltages that may cause harm to your camera.

Thanks Gonzogolf for that helpful information.

Happy to help.