I'm not sure whether the leaching issue has been really *properly* studied, in a tire planter sort of sense (that is, using oooold tires, as they tend to be). I know that the tide has turned in various directions over the years -- don't use tires, yay tires, don't use them, now we seem to be back to a lot of people saying they are probably more or less ok. I guess personally I would do it in newish tires but not in the crusty old ones that have been sitting in the sun for five years since they were taken off the tire (or at least, not to feed children from plants grown there).
Two things to remember: that warming-the-soil effect that's so great in spring can *cook* the roots of sensitive plants in summer, so you want to stick with heat lovers, especially if you do ultra tall towers like Rob's. And also, they will need a bunch more water than ordinary non raised non tire-encircled gardening, so it only works if you can and will put the extra water in.
I've never grown in tires myself, but I know plenty of people who have, and it generally seems to work well. Specially for things like tomatoes, peppers, eggplant, squash, melons.
Good luck,
Pat