I just wrote a post on my blog about those kinds of services, they should only be used when you know what you are doing. In particular, they can hurt you on StumbleUpon:
Social Bookmarking Can Kill Your StumbleUpon Traffic! | Caroline Middlebrook
After I wrote that people still weren't getting what I was trying to say so I wrote a follow up post and in that I explained some of the problems that occurred during last years 30DC when people got over-zealous with onlywire and similar tools without really knowing what they were doing.
17 Social Sites To Spam For Backlinks | Caroline Middlebrook