Almost never (less than 5%).
Before the iPhone I would occasionally play Tetris or something along those lines if I'm waiting around and don't feel like watching TV or if The Wife is watching something I don't really care for, but since the iPhone and iPad, I play those much more often at home than I ever played played my hand held.
Surprisingly, my DS still gets a fair amount of play when I go on road trips but my iPhone/iPad still get much more use. My DS only gets use on the airplane and airport while my iPhone goes with my everywhere.
And that's the problem with dedicated hand held consoles like the Vita and 3DS and why they're on their way out. The both beat the iPhone in every way in terms of gaming but the iPhone does everything else better and it's more portable (I can't fit my DS in my pocket comfortably and definitely can't fit the 3DS or Vita in my pocket). The 3DS and Vita can both try to compete with the iPhone in terms of other apps, but the iPhone is always going to win.
Even in terms of games, the iPhone is even coming close to the 3DS and Vita. It beats them in terms or pricing (free to a few dollars per game versus $40) and I'm pretty alright with not having huge epics like Uncharted on my hand held. When I play a portable game, I want something quick and simple like Words With Friends. If I want something with a little thought, I play something like Ticket To Ride.
And then there's the iPad which, in terms of games, is going to start blowing the pants off the 3DS and Vita.
