The site has been in talks for a long time and but its requirement of
adding a plug-in always kept me away. Plug-ins do make things easy for
you but they must not be mandatory.
Getting bored, I just created a log-in and downloaded the plug-in.
Sumbleupon has been quite famous and it was an instant hit for me. Got
me to some interesting sites. It is quite good for one of these days
when you have nothing better to do. But what all it offers in long
term is something to be seen.