I would certainly say that the Wii is more than just a fad. It has established itself as a worthy rival to the PS3 and X360; in fact it has destroyed them both when it comes to actual sales (how much of this is down to pure hype is up for debate).
A good friend of mine is a PS3 owner and is a strict first person shooter kind of guy. He plays Halo, Resistance: Fall of Man and COD4 and that's about it. I decided to bring round my Wii a few days ago to show him what it's all about. Before I even had a chance to show him Mario Galaxy he was hooked on Wii Boxing. He was actually playing with a smile on his face rather than intense concentration. The Wii brings gaming to a whole new level and opens up the door to new experiences.
I hate the term "hardcore gamer" and hate all the fanboy crap that goes on in the gaming circles. The PS3 and X360 may offer astounding realism but the Wii offers what gaming should be all about, and thatís all-round entertainment. The stigma attached to the Wii is a joke. So what if the whole family get's involved with the games? The Wii has already shown it's versatility to be both a solo gamers system (RE4, Zelda, Mario Galaxy) and also a brilliant way of interacting with friends and family (MP8, Mario & Sonic, Wii Sports).
In summary. I believe the Wii has already proven itself to be more than a flash in the pan. It has opened gamerís eyes to a new way of doing things. The Wii is far from the finished article but I truly believe it will be the benchmark for all future gaming systems.