<< The only good part of the deal is 0% interest for a year, the rest would be foolish to take them up on. >>
Exactly. So if you get a $25k car and put down $4k down, now they're financing you $21k.
If you put aside $300 month, you're paying off $3600 interest-free. At the end of the year, if the car is worth $19k (I'm using Sluggo's example if you haven't realized). You only owe $17400 + interest. If you can throw more at it per month, it's even better.
Of course what is the interest AFTER the first year? If it's something outrageously high, you're still upside-down.
Basically, the key is pay down as much of it as you can during that first interest-free year. If you just think of it as getting a car for free for a year, you end up losing out.