It's not as strong as the XBone and PS4, but more powerful than the 360 and PS3.
Just for the sake of accuracy, that's not entirely true. The Wii U utilizes a 45nm 1.24GHz tri-core PowerPC CPU, and is relatively similar to the 360's CPU, but about half the speed. It does have a larger cache available (sort of), and the GPU has considerably more memory available, but the lower clock creates a pretty substantial bottleneck.
Not sure what the instructions per clock are off-hand on the 360, but it doesn't really matter - while the "ghz doesn't matter anymore" saying is largely true these days, it's only applicable up to a point, like compaing a 2.8GHz AMD to a 3.2GHz Intel. A gap this large across a related system architecture is a noticeable one.
There's a 1.2MB L2 cache for one of the cores, but the other two can only access 512KB, which creates another bottleneck.
The sole area the Wii U is wildly better is in the available memory, which is a good thing to have, but the system takes a beating with anything that's CPU intensive, like particle effects, fill rate (think games like Zone of the Endes for a real world case), etc.
The PS3 is a bit tougher comparison, since the Cell architecture is wildly different than a PowerPC, but generally speaking, the Wii U would do worse with any games that were not specifically designed for multi-core processing and anti-aliasing. The PS3 doesn't do great with them either (Mass Effect is a good example), but its single physical core would do better than the one on the Wii U for those types of games.
