I do think it's great value at 35 dollars a day for adults, especially considering the amount of food you could potentially eat! But of course are you actually going to eat all of it?! AND the *big* problem I have with the Dining Plan is the fact that a 10 year old is counted as an adult. In what other context would a 10 year old *ever* be considered an adult? It's daft. I see from your signature that you have such a creature i.e. a 10 year old "adult"

So I reckon you'd lose money on her portion of the cost as she's unlikely to eat 35 dollars worth of food a day. But then it depends how much you and your significant other like your food. If you're going to eat mainly counter service meals and/or if you like to share main courses then I could see you losing money on it.
There seems to be many pros and cons. We're not doing the dining plan, mainly because we're going the Annual Pass route in order to avail of room discounts so we can't do it (you need a package). But the fact that I have a 10 year old - 11 by the time we go to WDW put me off it too. Well, having him is great but having him get charged 35 isn't. Damn him and his 1994 birth date. lol!!!
Then again a great 'plus' is the fact that gratuity is included!!
At one point I was thinking of all sorts of 'devious' combinations in order to use it (2 value rooms with DH in one as a 'room only' reservation with
AAA discount and me and the kids in the other on the dining plan etc. etc. - lol!) but we opted out.
Unless you're sure you'd all love 2 big sitdown meals a day incl. starter and dessert then I think it's a tough choice.
Good luck whatever you decide. We'll be there in June/July for the first time too. Have a great trip.

Oh, by the way, I love your username, very sweet
Edited to add: there are many posts about the Dining Plan over on the Restaurants thread. You'll get a lot of info browsing through them.