It is fine. You gets ton of traffic but ver little conversions. It is not against adsense rules. However, stumble exchanges are against the rules of Stumble Upon, so as long as you use it properly, you will be fine.
i've had several natural Stumble-storms bringing thousands of visitors... Hardly any conversions though... and, as far as I can tell, there is nothing in the tos or policies against it..
i've had severals article stumble at my blog and recieve like thousands visitor in a minute. but i don't see the adsense money increase.
Exactly, stumblers are not in a mood to click. They are mostly advanced users that want to find out about new sites and as soon as they're done with yours, they jump to the next one without clicking! So no conversions at all! Or almost none...