I would only assume the PayPal donate buttons work for loyal visitors. If you have a blog, game, forum, etc-- the ones who come back day after day would only consider donating.
I used to think the looked like begging but now they are more common...the "buy me a beer" or "buy me a coffee" is a nice way to dress it up.
Ive donated through those buttons a few times but only for sites/people that really worked for it and didnt have other revenue streams. Specifically podcasts that I regularly listen to or a free software tool that I ended up using a lot.
In most cases they're pretty much worthless, and a waste of time. I had buttons up on a site for an open source project for 4 years, it was very rarely used.