I could have bought my last car with 0% finance from the main dealer but the trade in I was being offered on my own car was a lot less if I was going for the 0% finance offer.
Without a trade in, 0% finance was only available at the list price, whereas a discount off list price could be got if buying without using the dealers 0% finance offer.
In both cases for me anyway, 0% finance was still costing a lot. Like everything you have to crunch the numbers, 0% finance might or might not be a good deal..