Nintendo has announced delays in the launch of several highly anticipated titles, including Wii Fit U. We did wind up buying one and spent this weekend exploring the capabilities and playing some downloaded games. Only NintendoLand (included in the 32GB "Deluxe" version) so far actually takes any real advantage of the capabilities of the Gamepad controller, but if that is any indication, it could prove quite interesting.
It looks like there are enough titles to keep it from being an expensive doorstop, unless you are a hardcore gamer that plays every title available, but I don't think it is aimed at that audience.
And it is backwards compatible with Wii games. It will also transfer content and settings from a Wii, but the process is not reversible. Once you perform the transfer, they are gone from the Wii and can't be restored. This could be a problem if you wanted to use the Wii U in one room and pass the Wii down for use in another room on an SD TV, say.