I used to play Mario Kart DS Wifi alot. I was actually really good at it too, managing to hold my own against the people considered to be pros back then. I eventually lost interest.
Quoted for... uh, me feeling the exact same way.
I also got MP:H, and enjoyed it for about a year and a half before growing tired of it despite being able to hold my own against most pros on a good day. On random though, 50% of the time there is an AR user (even moreso now, Random is a joke) and the whole thing was like a stupid "make the most kills" melee thing so people playing Sylux or Trace could jump in, score an OHKO with Tribomb/headshot (well, near OHKO in Sylux's case), and then run for the rest of the match. Same with people playing as Samus and spamming the fucking Morph Ball to death, except you CAN'T catch those fuckers. Stupid shit was stupid. Also people disconnecting was really a tremendous piss-off. Nintendo should learn that the only way to deal with disconnectors is to ban... uh, somehow.
Since those two games, I haven't gotten one that's online enabled though I'm greatly looking forward to Brawl (which I will never tire of) as well as Mario Kart and Battalion Wars 2. In terms of DS games I'll probably start playing Zelda: PH once I get it, as well as the Fire Emblem 11's online mode, whatever it may be.