You're not missing out on much with Wii Play or Wii Sports iPPi, they are pretty much just demos, both are fun for a few plays with friends, but thats about it. My roommate and I are looking forward to Smash Bros Brawl, since we've had our fill of Mario Party 8, WarioWare, and Rayman.
Also Ghandi, my roommate owns a Wii and we hardly ever play it unless its a multiplayer game, we rarely ever play single player with it, since theres not a whole lot of good single player games out for it, besides Zelda and Metroid. If you plan on playing with friends a lot, it's a blast, but by yourself most of the games aren't as fun, but that's just me.