I just don't see myself using Wii fit enough to justify the purchase. I got Wii Motion Plus because I forsee wanting it for future games mainly.
I have gained 7-8 pounds since I started dating my current GF. I take her out too often.
I wouldn't count on it being useful for future games, the history of console peripheral add ons being utilized in the long term is not good - essentially if you're a game developer, do you make your game for the thing everyone has (the wii-motes) or the thing only 10-20% of your market has (wii motion plus)
very rarely does the extra peripheral catch on...
see: Nintendo Power Pad, Nintendo 64DD, Sega CD, PowerGlove, etc.