I signed up to StumbleUpon a while ago, but have only recently started using it in earnest. I don't think I am deriving the full potential benefit yet.
One benefit is that I constantly run into articles or other web content I am interested in. The clutter up my browser's bookmarks, so this is a good place to keep them separately, and come back to read them when I have the time.
Another benefit is that by using the "Stumble" function you get random articles or sites that may (or may not) be of interest to you. As you use it the program progressively determines your tastes and interests, and (ideally) would be recommending content more and more to your liking.
A third benefit is through your friends lists you Stumble (or just view) articles that they are interested in, and hence "recommending".
So far I have mainly derived the first of the above benefits. The system still doesn't "know" me well enough and a lot of the content that I "Stumble Upon" is not of interest. I also have not added many friends yet, but am trying to increase their number. Maybe when that reaches some critical mass the third benefit will kick in more.
This is all mitigated by the fact that I really shouldn't be spending so much time reading stuff on the internet! It can be endless.
(This post was originallly a message I sent in StumbleUpon to my uncle Sanjit Sengupta, but I thought it summed up my thoughts on StumbleUpon sufficiently to share with others.)