I think it depends on the game.
Wiimotes are great at games like Fighters or Sports games, where you play in short bursts and moving one's hands in immitation of what you want the game to do makes sense. For other stuff, it just doesn't work.
Tell me -- What's the intuitive way to move the nunchuck/wiimote in a strategy game (Warcraft)? It's not nearly so easy as thinking swing the wiimote = swing the bat in MLB2008. Or using the nunchuck/wiimote as the player's hands in a boxing game. It doesn't make as much sense for sims, and somewhat less for RPGs.
Second thing, what about games that you play for hours at once. How long can you play a FF game using a Wiimote without a cramp? Sure you can swing the bat for an hour or so before getting tired, but imagine a 5-hour gaming session of an MMO or a 3-hour session of a RPG. Your wrists will get sore.
I don't think Wii is a gimick, but the remote isn't going to replace other controllers.