Here is the problem with retail tax chains (in general, certainly not all), they often hire directly from the tax prep classes they provided. While it is good that these people have some education, chances are they have a severe lack of experience and in my opinion that is not something you want. Additionally, some of these retail chains charge exhorbinant fees because they charge per form. So it doesn't matter if you have the most experienced staffer preparing your taxes or the least experienced, the charge remains the same. If you choose to go to a big box store ask for a senior preparer, but you may want to call ahead and make an appointment since these people often have a clientele base and may be booked/busy that day.
If you have a complicated tax situation beyond your mortgage and four kids, it may well be worth it to find a preparer who has had at least 5-7 years of experience. And has also worked outside of a retail tax chain. Honestly, with what some of these companies are charging, you can probably have a CPA, EA or other well qualified person for the same money, possibly less.
Here is a good article about preparers to avoid.
Your Mobile Tax Pro,