There seems to be a lot of confusion as to the power of this console when compared to the HD twins. Now we have another article claiming that the Wii U's GPU is very similar to a Radeon HD 4890 which is a very good graphics card for PC.
Link to article
As you can see in the video above, it runs Crysis 1 at 1080p with 8x FSAA and 16x AF. That is rather impressive. Taking into consideration that IGN ran an article before stating that developers with dev kits mentioned that the Wii U's GPU was based on a Radeon HD 4850, these claims may turn out to be true in the end and the cost is right for Nintendo to mass produce these chips.
Remember that Nintendo has been modifying this Radeon HD design for the last few years with new technology and research & development costs. The GPU that will be powering the Wii U in the final dev kit will give performance much greater than a Radeon HD 4850-4890 based on the modifications, optimizing and for the fact that it will be in a closed box console built for gaming and not a PC with an Operating System and background programs running all the time.
The article in the link suggests that the GPU powering the Wii U is "last gen" but still more powerful than 360/PS3. While this is technically true on the architectural base of the GPU, the final product will be miles ahead of either the 360 or PS3 by a long shot. It is looking to be more in the range of 4-5 times the power of those systems.