Here's a prediction/analysis I wrote based on someone's comment that the Wii U will sell as well as the Wii. I'm willing to stick by it.
If you're saying the WIi U will follow the path of the Wii, you have to look at why the Wii was successful.
1. I don't think anyone can deny the fact that it sold a ton to "casual" gamers — and even non-gamers. Soccer moms that picked up a Wii to stay fit, maybe bought Mario Kart or NSMBWii for their kids, then never bought a game again. Nintendo is currently not appealing to this audience. They've got few casual titles lined up, and even if they would create them, the Wii U's appeal is less immediate than motion controls. Oprah isn't featuring this thing. They may feel they wasted money on the Wii based on how little they used it, so why would they buy the new one? Plus, they've all got their tablets and 5-minute-games. I'd ballpark this group to be 40-50 million Wii sales.
1B. (Small group I'd add to this: people who loved Nintendo when they were kids, are now too busy for games but may have picked one up for NSMBWii or something. Nintendo can appeal to these again, but I don't think it's a huge group.)
2. Second group is Nintendo fans. That's you, the person reading this. You've got a Wii U or are planning to buy one when good games start rolling out. There's about 30 million of us, it seems, based on N64 and Gamecube sales.
3. Third group's the hardcore gamer that may have picked up a Wii cause it was so cheap, and that Red Steel game looked pretty promising. You then sold your Wii, or you're one of those annoying people complaining about how it "collects dust", while not buying Little King's Story, Madworld, Donkey Kong Country Returns, Sin & Punishment, Red Steel 2, Xenoblade, No More Heroes, Last Story, Metroid Prime 3 or Okami. You probably didn't buy Super Mario Galaxy 2 because the highest-rated game of all time looks kiddy. Or you never bought a Wii at all because Nintendo is kiddy, and you never plan on buying a Nintendo console again. Nintendo has got some interesting games for these in store, most notably Bayonetta 2. But we want graphics, grrr. We want manly violence, and we want EA sports games. Not getting many of those.
Conclusion: I think Nintendo has to stunt big time to sell more than 40 million consoles. They're not appealing to the mainstream, and they're not appealing to the hardcore (sadly you'll need better graphics than that). Their main trump card is that they can sell the Wii U for way below the price of a PS4 and Xbox One, making it more likely for people to pick it up as secondary console. They still need to put some effort in to sell to Nintendo fans, but I have no doubt that they will.
As a Nintendo fan I care very little about all of this; I only care about fantastic first party games, hopefully some quirky 3rd party games (a la Wonderful 101), now with more indie goodness than ever.
TL;DR: I predict the Wii U to sell 40 million consoles.