My sister really likes it. She's playing the hell out of WiiSports and she kicks butt at boxing. She's played games before, but generally only with me; I suspect she'll play Wii on her own time, like she does sometimes with the DS.
My mother and father both really like it, too. My mom was impressed with how much of a work-out boxing was. They both played it, and were enthusiastic about playing it; we bought two at launch, one to keep and one to sell, and I think we may end up keeping both now.
All my younger cousins loved it a lot, although that's not particularly surprising. I convinced them to buy a Gamecube, which they liked. The older, Xbox owning one really liked the Wii too, though.
Even my grandfather played it with me, and called it "neat" and seemed to enjoy it.
Played it (WiiSports) with my friends a few days ago. Two of the three were really casual gamers, and they usually only play games with me and my other game-playing friends; mostly Super Smash Brothers and the like. The third refused to play Wii because he already intends to buy one and didn't want to tempt himself into buying it before he had expendable money. My one friend was fascinated by how it worked, and was really impressed with it, enjoyed it thoroughly. The other didn't express such a fascination with it, but he definitely had lots of fun, and he's not even into sports at all (none of us particularly are, yet we all liked WiiSports).
Haven't shown it to any of my friends at college yet, because I didn't bring it up (lack of TV), but I'm bringing it up to show them next week, probably. They all have a mild interest in it, ranging from "Oh, I heard about that" to the one who's a mild gamer (definitely not casual, but far from hardcore) that's pretty excited about it; he's a bit of a Nintendo fan.
I've had no bad response to it yet.