I just checked my refferring links, and I am getting about 10 people from Stumble Upon per minute. It came randomly, starting at 12:40 am today. I submitted individual articles a few times with no success and added about 50 friends one night, but now that someone else has submitted my site, I am getting a ton of hits.
Yes, stumbleupon is great for traffic! My experiences with SU have been good, which is also why I started up my stumbling service, to help other people get traffic
Yes I got 1000 uniques last night for a single article stumbled, I am really happy with SU, also the traffic is huge when u get featured on the tags.
I have one site that seems to get put into SU's rotation every month or so. I get high volumes of traffic for 3 to 7 days and then it dies. Unfortunately, the site is one of my least money makers. Lots of traffic only results in a small number of clicks. Interestingly, I have two other sites that I promoted in pretty much the same way on SU. They both got an initial surge or two but then died and haven't ever gotten any more SU traffic. I guess if your site doesn't receive enough "I like it!" votes during the initial surge, then you get knocked off the rotation list. Too bad the SU links have the nofollow tag because if you don't get raw SU traffic, all your promotional work is wasted.