Well I tried playing Star Fox Command online the other day and couldn't find anyone. I played some Metroid Prime Hunters back a few months ago and there seemed to be plenty of people playing but I didn't feel like learning the rulesets, level layouts, and item locations, so I only played the one time.
I tried playing Clubhouse Games at release and had no trouble bowling or playing chess, but everything else took forever to find anyone to play. I played some Mario Kart and won a few rounds before I found a snaker. All this game needed was Wifi battle and I'd play it everyday, but for some reason they couldn't do that, snaking would do shit for those people in battle mode.
Honestly, I'm not having much fun with the DS wifi connectivity, I just wish I knew more people with DS's so I could play locally, which is always really fun.
Oh, and I can always find people to play Tetris, which freaking rocks, btw.