Is it worth using affiliate links or does that harm you in the eyes of Google? I know some use them all over their site. Do you have to limit how much you use or how many to prevent being penalized?

I heard that they can do your site's PR more harm than good and am cautious about implementing anything that's going to reduce my PR.Is that true or is it a myth?

Please share your ideas on whether affiliate links are a good thing or bad. Thanks!