Yes, I have used it a few times on a small scale ($20 a few times). The traffic comes in waves - they'll send a bunch of traffic and then it drops off for a few minutes and then comes again. Make sure your site can handle 20-30 simultaneous visitors.

The quality of the visitor (in my opinion) is poor. Although you may have filtered it down to a group that you think is interested, remember that this is someone who is clicking a button to see a random page. It's highly likely that they're going to see your page and click the random button again.

If you're just looking for traffic, it's a great, cheap way to get some. If you're trying to sell something, I'd recommend something more targeted and traditional, like a facebook ad.

One thing I did notice: In the weeks and months following a paid campaign I would occasionally see an 'organic campaign' - a sudden rush of organic SU traffic, several times larger than the paid campaign, for no good reason.