I'm a HUGE fan of social bookmarking and have done many tests with using only SB on brand new domains to see very big gains in SE rankings for my targeted keywords.
People who say it doesn't work either:
(a) only used nofollow sites
(b) didn't wait long enough for Google (or Yahoo, etc.) to index the pages where their bookmarks appeared, or didn't ping those pages
Plus the new Google Caffeine search algorithm seems to favor social bookmarking even more than the current algorithm. Every one of my niche sites I've checked, which has a lot of social bookmarking backlinks, has jumped in rankings. See for yourself:
Compare Google Caffeine