Digg, Stumble, Del.icio.us,.... do they really work ?

I've recently seen really many members requesting from others to digg, stumble or del.icio.us their site for them

Never really tried placing any of my sites on any of them, but do they really work in grabbing traffic ?

To be honest, I've never seen anybody using these services except webmasters promoting their sites.
Do these sites get alot of traffic from regular users ?
And are they really worth all the time and effort spent on them trying to get dugg or stumbled or whatever