According to the leaked specs Project Cafe/Wii 2 could potentially be a lot more powerful than the 360 or PS3, but it depends what versions of the hardware they actually use. The processor is supposed to be a 3 core PowerPC, similar to the Xbox 360's but with a faster clock speed. The GPU is based on AMD R700 architecture, which at the top level could exceed the PS3's GPU by about 4 times and even the lowest model would probably have twice the performance of the 360's GPU.
I wouldn't expect the games to look much different than current 360/PS3 games though (at least in screenshots), the only difference likely being a better framerate (hoping for 60fps as opposed to the 360/PS3 standard of 30fps) and displaying at 1080p (some games on 360/PS3 go as low as 540p, such as RDR). That would make the games feel much smoother even if they didn't look drastically different. They could also have improved lighting and effects but I doubt it'd be too noticeable to most people. 3rd party games will look pretty much the same on all the platforms (I'm pretty sure that Ubisoft is planning to port everything from the 360) but Nintendo's own games may be able to do a bit more than what we've seen on the other consoles.
Expect the online capabilities to be far improved over the Wii. The 3DS has already shown that Nintendo knows it was a weak area and the new handheld has much better connectivity (such as the Streetpass and Spotpass functionality); so I think that they'll try improving on it further (I'm betting that friend codes will either be abandoned or made to be system only rather than for each game).
So is Nintendo's new console going to be worth buying? If the hardware matches those expectations and it hits at the expected $300-$400 price point then I would say that it's a definite yes. The reasoning behind that is that neither Sony or MS will be able to produce a console that is much superior to 'Wii 2' within 2 years without it costing around $600-$700 and it's also a much harder sell as they already have 'HD' consoles on sale, unlike Nintendo.
I think that Sony and MS tried going too high with this gens hardware by making it too expensive initially and tried to counteract that by doubling the consoles lifespan (Sony in particular said that the PS3 was a 10 year plan). Nintendo stuck with the regular 5 year plan and it looks like working out much better in terms of affordable technology.
I guess we'll find out at E3 though.