I use them, every time I publish a blog I will bookmark it there. As I use the onlywire FireFox plugin, it posts to the majority of my social sites automatically. I really have not seen a ton of traffic from either site, Stumble has been working the best.
One thing it does do though is give you a backlink and I will form time to time see someone coming from a Technorati site. If you use Onlywire, then it is no more effort and the backlink will help. If I were submitting manually, I would most likely skip those two as they have the least traffic to me.
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"People who are unable to motivate themselves must be content with mediocrity." ~Andrew Carnegie~