There is a small difference between the visitors from PPC and from SU. In PPC ads, the visitors that you get are proactively looking for a specific solution and hence the conversion ratio may be higher with such traffic. But you know the costs!
The traffic from SU are not as needy as the former ones, but you can expect reasonable conversions from SU traffic provided that you have make them an offer that they cannot refuse. It works particularly well for lead campaigns where the visitors do not have to pay anything right away.
SU does not allow you to put sales pages in their system. Think of SU ads as you would think about article marketing. You post articles in ezine articles and other article directories which gives away free information and then redirects the visitor to your sales page. In you want to leverage the SU system, you have to think in a similar fashion.
The cost of advertising in SU is fixed at 5 cents per visitor, but if your content is good, many people will give a "thumbs up" for your content and you can reduce the cost to 2 cents or even in some cases just 1 cent per visitor.
In my experiments with stumbleupon, I wasted a lot of money in driving traffic to pages which were in some way obviously promotional. In such cases I got no conversions and no residual traffic.
Then I posted a video which was very informative and in the end of the video it asked the viewer to visit a particular URL if they needed more information. Many visitors were generous enough to give a "thumbs up" and I started getting residual traffic from that campaign. See the screenshot below, right now the effective cost per visitor is just 1.78¢ and it will go down as I get more residual traffic!
The blue bar represents paid traffic and the green ones are free. I paid $5 two times for the initial boost. Once you start getting positive votes, then you can leave it for a while.
This traffic is for a youtube video. Even if the traffic from SU stops completely, my youtube video with a few thousands views is always a great asset. It will start getting traffic on its own.
The best thing about SU ads is that you can test it with a few dollars. If you get some positive votes, you know it works and then you can scale it up. If you do not get any residual traffic for your campaign, drop it and test more.
And the URL is www.stumbleupon.com/ads/