The only phones any Radio Shack or Best Buy I've ever been to have are non-functioning, fake demo units, and are probably not a good indicator of build quality or durability.
My keyboard has become slightly less stiff since release, which is a good thing (it is really stiff out of the box--especially if you are using a case). Today, it is by no means loose nor does it give any when it is in my pocket, etc.
I would go to a Sprint store and check out their working display unit. I think you will find the keyboard remains in good shape.