Worth the money? Depends how you look at it. It's definitely not used for serious gaming.
The graphics on the Wii is powered by the same video card on the Nintendo GameCube, an ATi Hollywood GPU. The graphics you get on a Wii will NEVER be as great as what you can get with a PS3 or XBox360.
The wireless on the controllers is sensed by a little bar. The controllers themselves are implanted with an accelerometer and optical sensor technology. The controls for the Wii can frustrate many people who have a hard time controlling precise movements with their hands. They have a limited range, so you have to be directly in front of that sensing bar. I know I've almost drop kicked a Wii because of it's poor sensing ability.
Another downside to the Wii controllers is that they have a tendency to fly. All it takes is a little bit of a loosened grip, a little hand sweat, a little swing, and all is but lost to the flying Wiimote.
The Wii doesn't come with a hard drive unlike it's other 2 counterparts. You lose a little bit of that one-in-all with the Wii. However, it does offer old nintendo classics on the online menu. If you were an old school, retro, classic, or any other descriptive term to depict ancient games, gamer, then the Wii might just be for you. You can find classics like Legend of Zelda: Link to the Past and Super Mario World in the store. Only problem is, you need a GameCube controller or the special controller to play those games.
The Wii does offer backwards compatibility, so if you have old GameCube games, you can just throw it in and play the games. It'll even take the GameCube memory card.
Overall, I wouldn't get the Wii. You can find whatever it has to offer elsewhere. From a computer emulator, to actually playing sports, there are better things that offer more.
If your fiance has an XBox, offer to go get one too so you can both game online with him. It'll make his day more than a Wii can or will. Otherwise, go for the PS3. The controllers have motion sensing capabilities also.