ParkShark was a parking space sharing iOS app. The technology was sound but the market timing was a little early. We were even talking to some larger delivery companies about a business-side component. The product failed because the two founders, myself and a friend, are engineers through-and-through. We just didn't have the passion for the business side of things to really make it happen.
You're doing it right for your application! MySQL or PostgreSQL would most probably be slower and introduce a lot more overhead as they are client/server oriented systems. Don't listen to those armchair architects!
- Thomas Jefferson