You can get a few visitors if your comment is one of the first ones, plus it should be interesting enough to make people want to click through to your site. I'm not sure I'd call it nice traffic but it's a start.
It can work, but like MTbiker said, post something interesting. Also, be sure to post comments on blogs with simliar content to your site.
Well... you gotta make bloody useful comments. It's not like going to give you great number of traffic (more like just a couple of clicks - if any) - and when you get some quality people interested, they might spread the word. So yes - it works. But requires quite a lot of work.
Add the blogs you want to comment on to your RSS reader, that way you know when they update and you can be one of the first to comment on them. I wouldn't call it great traffic either, but if you run a good website it is a good way to pick up some new RSS subscribers. And try to make sure it is related.
When you visit a blog, click the little orange icon you see in the browser's address bar (to the right of the URL.) Then add it to something like http://www.google.com/reader
shameless plug - i currently provide a service where i provide you with a list of blogs that have "do-follow" tags in the comments. $10 for 5 keywords. check my thread
yes it would help a lot but make sure the comments are dofollow you can use the following tools to search for dofollow blogs for your keywords http://w3ec.com/dofollow/ http://www.ezbusinessneeds.com/dofollow.php