We won't know until the end of the month when Apple will give us our financial reports. I wish we could find out sooner. I could have sold 50, 500 or 1,000 apps - I just don't know!
Hopefully for many of us it will be like waiting for Christmas to get those reports!
I take it back...Medialets caches stats on the device as well (their web site isn't well-designed with respect to finding information easily). They also have an advertising delivery mechanism, however, and it's not clear just how detached that mechanism is from the stats api's. My biggest concern with them, then, would be from a privacy standpoint.