Bookmarking
For me switching to an online bookmarking tool (though stumbleupon is more than this) was an immense relief. Bookmarking was selecting to bookmark and then selecting a category which fit (often poorly).
Now all I have to do is click the thumbs-up. Almost any time I want to rate a site another user has been there before and tagged it. Therefore it goes into my favorites in the correct location. If I do happen to visit a site first then I get to know that I'm helping other people not to have spend energy in the future.
Unfortunately stumbleupon does keep your favorites. I haven't seen a way to export them.
I tried using other services such as del.icio.us but found them more cumbersome.

Stumbling
Stumbleupon is mainly built for stumbling- visiting sites that other people have approved of. I haven't done as much of this but when I have I've found that the sites it suggests are entertaining.

Great fun for the first couple of hours. Sometimes you are just in the mood to see new stuff and I have yet to see a product that does a better job of giving it to you.

The neat thing about Stumbleupon is that it can introduce you to sites or apps that may be outside your normal lens. Even if I get onto digg, I'm still looking at sites that match my interest, while stumble will mix in some very different content.

Stumbling is a great way to find new sites. I didnt understand it at first but then relaised how powerful it was. If you have a website and someone reviews it on stumble upon it can drive a lot of traffic to your site.

I am a regular stumble upon user and the toolbar makes it easy to use everyday.

I like the idea with StumbleUpon but I don't like the implementation. It seems cumbersome. Now, I don't pretend to know how they should change it to make it more intuitive, but they need to change something to make it easier for the average user.

But all in all, a great tool that is invaluable whenever you think you've seen it all on the web.