I have doubts over the Wii U achieving success, at least anywhere near Wii success, but what I don't have doubts about is Nintendo continuing to make quality games that I enjoy. While I'm not completely sold on the tablet, that won't make a difference to me as long as the games are great. And actually, I do think certain genres such as RPGs, RTS, etc, would work great with the tablet control.
Sales wise, I haven't really seen the same qualities the Wii had that brought a mass market audience. Nintendo seems to be going back to the Sony and MS style of games they went with on the N64 and Gamecube, which could alienate their Wii userbase. But at the same time, I think those lost mass market gamers will be somewhat offset by an addition of more core gamers and better third party support.
