My Halfling Cavalier is one of the few I've seen, but then again, my playgroup isn't that large. I know of one other Halfling Cavalier (I think, I've seen him play, but never played with him), and a Gnome Paladin that took a wolf for a mount for mobility reasons.
I specifically took a Halfling because of the Medium-sized mount. Didn't want to get stuck in a 5-foot-wide corridor.
I must say, it took me a while to get my mobility to where I wanted (took Wheeling Charge at level 7, bah), but from then on, my damage output is great. I disagree with Jayson, I keep out of melee range and manage to position myself right for precise hit-and-run strikes. Sure, it's a shame I don't get my iteratives, but 3d6+12 (more if I challenge) is more than enough for most enemies, considering others are already weakening them. And as stated, if I don't kill them outright, I can just run away without any trouble.
Also, I took Monstrous Mount as my level 5-feat, just so I could add a little extra to my class. You don't meet very many characters who can say "yeah, I ride a Worg." :)