When I first bought my Wii U I really thought there was something wrong with it. It took over 30 minutes to set up as it was sooooo slow changing from one screen to the next. I reset it multiple times before coming to the conclusion that I had a duff console. I had tried system updates and everything to get it to speed up, but the reality was that the new Mario U took over 5 minutes to load after seeing the title screen.
I was literally just about the return it to the shop one day when I noticed an update so I gave the console the benefit of the doubt and updated it. It's now resolved. Going from one screen to another takes less than 5-10 seconds at the most and it's super fast.
The update time depends on the last update like any console. If you haven't bothered to do an update or put it off for a long time, then it will take longer to update because there're more data to download. It also purely depends on your connection speed too. I have super fast broadband so, for me the updates are quick.
I'm digging that you can play it with the handheld divide. As I'm a Mum of 3 children, when the TV is on, you (or the kids) can still turn on the console and take it around the house with them wherever they want to play it, and I still get to watch whatever I was watching on the TV!

I've also played on the Wii U in my bedroom when the console itself is located downstairs and even took it on the garden too.
The HD alone is worth its weight in gold. The screen is soooo very clear. My daughters love the Wii U chat where they can take the handheld device around the house with them and chat to their friends. i don't use this feature, but it's pretty darn cool! They love watching YouTube music videos on it too and generally browsing the net.
And I love that it's got backwards compatibility so they're able to play the masses of standard Wii games they accumulated over the years with the regular Wii. Although if you're playing regular Wii games, you cannot use the handheld controller and have to use the TV too.
If I had to rate it out of 5 I would give it...
★★★★☆
[EDIT] Didn't realise this was an old post. I'm looking forward to the new Mario Kart though. Looks awesomesauce!