Every stat I've seen says the Android tablet market is Nexus 7, Amazon Fire HD and everything else is fighting for scraps. Can't say for certain of course as Samsung won't tell anyone how many tablets they sell, just how many they ship. However I believe if they were selling loads they'd want to tell people.
(We may have to discuss whether the Note is a tablet or not. I'm kind of ambivalent).
I think it's fair to say the Nexus 7 is hurting Apple, but it's hurting Samsung just as much, if not more.
And there's of course the continuing argument that Apple don't mind losing market share as long as they make the majority of the profits. The Nexus 7 may be selling loads but I suspect it's not actually making anyone very much money in doing so.
Note: I have a nexus 7 at work, it's a ruddy good piece of kit. So I'm not considering product quality at all, just simple economics and business strategy.