Merc,
I would not say they are a good backup but are very challenging to find. I've only bought one box which was a skunk. I mostly do bags and handrolls.
I haven't searched enough quarters to say one way or another if boxes are a good way to go. My experience with boxes of nickels, dimes, and halves have been that I find older and unusual coins in them,but I skunk out a lot. I do better with bags as far as keepers go. I do alright with handrolls but find alot of foreign with the dimes and Nickel short rolls with the quarters. You have to be a real cheapskate to stick nickels in the middle of a rolls of quarters. I would love to run into someone pulling that. Heck, I'd introduce him to "Mr.Hand".
I still have one town in mind that I'd like to do a large quarter search in. Maybe next week.