Yes, if used right, they can help you very much. They do require however that you have a high quality/interesting/funny site, so other people get inclined to digg/furl/etc. it.
Very much. I benefit on both sites as I do bookmarking.. They bring good and quality link development.
A tiny bit, it certainly helps with getting a site indexed more quickly and to make your backlink profile looking natural, but I wouldn't waste too much time on it. SY
Has nothing to do with the fact that you can get a link on the sites. The point is, social bookmarking sites offer a platform with which you can promote extremely high quality blog content, which can then be linked to be a large crowd of webmasters/bloggers who pick up front page, popular content. This is the best type of link building.