Is it bad to have flat feet, and is there a good treatment for flat feet?

Replacement insoles: The question is quite vague, but i'll assume you want to know about arch supports. These can be custom made orthotics, made specifically for your individual feet, by seeing a podiatrist. For otc supports, i really like spenco medical products. Try the polysorb cross-trainer, walker-runner or total support styles to get more support in your shoes.

It is bad if it : Causes pain. Flatfeet are also more prone to causing biomechanical "faults" or problems. Conservatively, you can try orthotics and see if it helps. Check out fix myflatfeet.Com for an interesting surgical procedure with minimal recovery.