Handheld sure, but I personally think they are screwed as a normal platformholder next gen. The big problem for Nintendo is that they are now aiming at the core gamers as well which takes away the family friendly image that the Wii has. Another problem is that it's only 40% more powerfull then current consoles and will probably be a lot more expensive when it launches.
The quesition is if the Tablet Controller is enough to really push them through and I think that will come down to the support Nintendo can get for it. Their controller requires specific games designed for it else you just get some shoe-horned in functionallity, meaning multiplatform games will be pretty much identical on all 3 consoles. Considering the Wii-U has to play catch-up with a 140m installbase of MS and Sony combined, I don't see a lot of 3rd party's supporting it in a way that really makes the Wii-U version of their multiplat games the clear superior version. IMO it will all depend on Nintendo's first party games and if people will buy it for the multiplatform titles as well. For example how is the online service compared to XBL?
What I do know is that 40% more powerfull is way to little to seriously compete next gen. In two year time we will probably see MS and Sony launch a console that is 10 times more powerfull and it could be the Dreamcast all over again.