Comment etiquette

Just been thinking about this lately and I am curious how people think about it… if someone (you don’t know) posts a nice comment on your site, what do you think about saying “Thanks!” Do you post back in the comments, do you send a quick email (assuming they left an addy), both, none? If you just post a comment, it’s highly likely they won’t be back to see it. Would it be considered rude to just email something like that, is it unnecessary? Obviously if they didn’t want emails they could just not leave the address (unless you require it). I can see that some people if they visit a lot of sites and leave a lot of comments would get tired of receiving a lot of little thanks emails. But I also find that sometimes getting something nice like that, showing that people are still considerate and do appreciate what you had to say can give a little warm fuzzy in the day.
Thoughts?