I haven't seen much positive impact from them either way, honestly. Delicious popular page can send some traffic, but that's about it.
True enough. I am mainly using social bookmarking sites to bookmark those pages that I want giving solid link juices to the site I optimize. Ex. I guest posted on other blog. I would then go bookmarking my guest post to add weight to it to make it a solid contributor of link juice.
I really only focus on Digg.com for 'social bookmark'. I get a lot of value from Facebook and Twitter, but the purpose isn't to build backlinks - it's to build traffic and awareness. Honestly, if you write good articles, you can generate quality backlinks just through others posting links for you. It's all about starting off with good content.
Depends on the keyword phrase, competition, how popular your site is to begin with, and a ton of other factors. Social bookmarks would be most helpful if they are dofollow keyworded anchor text. There are just so many factors it would be hard to say. No sense bookmarking a page that isn't really that informative...like what you had for breakfast. Not cool. Just bookmark them and move along. It's not something to spend all day with.