If I got Wii Fit I'd at least give it a try. I'd probably keep it since it's a Nintendo game and all. I doubt I would keep Wii Music though or DK Barrel Blast (ugh).
I can trust my family not to do this to me. The general rule is that if a videogame is not on my Christmas list, don't buy it for me. Ditto with CDs. DVDs are a little more open since I don't specifically collect them and thus am not so picky about. My parents know me well enough to not do this to me.
My immediate family is pretty much the only people I get gifts from anyway. My friends and I never bought each other gifts, I'm currently single, most of my grandparents have died, when I was a kid my aunts complained that buying for all the cousins was too expensive so I don't get gifts from that part of the extended family, and we don't do a Secret Santa at work anymore.
The only thing I have to worry about is birthdays as my friends and I do buy birthday gifts. A friend who should know better bought another friend an unrequested videogame for his birthday and I was concerned he would do the same to me. He didn't, thankfully, as he doesn't really read reviews anymore and with the Wii you really don't want to buy games based on the cover art. "Hey Sonic was great on the Genesis, I'l buy Ian this!"
It's like a different take on the "clueless parents buys shoddy game based on popular licence." Clueless friend buys shoddy game based on videogame franchise that used to be good.
