StumbleUpon is a curious mix of social bookmarking and social networking. It is based on quite a unique concept. I consider it a bit of a hybrid among social media brands. But it has its negative points as well as its positives.
On the positive side, StumbleUpon is great for driving new traffic to your website. On the negative, most of that traffic bounces.
Unless you have something that SU visitors really want to see, don’t expect them to visit more than one page on your website. They’ll visit, and they might even give you a thumbs up (many SU users actually give you charity thumbs up in hopes that you’ll do the same for them), but don’t expect them to stick around.
Is that good? It depends on what you mean by good. It’s bad only if you value a high bounce rate. But the real test is, does that StumbleUpon traffic make you money? If it does then you’re gold.