You can get bad value if you're in a rush. Normal priced items can be a bit uncompetitive price, if you're in no rush usually in a month or two a deep discount comes up.
For example I'm more interested in power density and input speeds so the new Powercore 26800 with 4A input peaked my interest but I own plenty already so no way would see the value of $60 but happy to be tappity tappity at 9am PDT for the 50% discount and got my Powercore 26800 in today's post. It adds to the 3400, 5000, 10000 mah products I already have.
Actually what I find wears out fastest is USB cables, as they tend to get coiled up and used most often and they tend to fail faster than the batteries so I find I'm more buying cables and bought the newer ones to simply last longer.