I'm a vegetarian and have used the DDP twice. I've never had a problem finding a vegetarian meal in any Disney restaurant--there's always at least one entree on the menu for vegetarians. Most of my meals have been great, a few just okay, but this is true of any meals, veggie or not.
Money-wise, both of my DDP trips have been on
free dining, so the cost wasn't an issue for me. But if I had been paying for the DDP, I'm not so sure I would have been willing to buy it because, as Tricia pointed out, vegetarian entrees are always the cheapest things on the menu. Of course, at buffets it doesn't make any difference, and I travel with my dh and 2 kids, who are not vegetarians, so as a group, we likely would still have saved money with the DDP. But the decision whether to use it or not would have been more difficult.