I just picked up Excitebots and am really loving it so far. No surprise, since I was a big fan of ExciteTruck, and this one seems better in many ways.
However, the vs. mode is disappointingly stagnant. It still maxes at two players, and you still can't include AI racers.
I've barely played online mode, but it looks to be pretty thorough (except for WiiSpeak, d'oh!). Finding a random match was pretty fast, and the betting system looks interesting.