Unless you have lots of time on your hands and the patience to sort through all the material available, Ancestry may not be your best investment. It's primarily a resource for people doing family research, so I would caution against acquiring it if you wish to access military records. On the other hand, if you would like to contact other people who happen to be working on related family lines, it will prove to be an excellent resource.

In any case, try it at the public library or find a friend who has Ancestry. The you can decide if it's worth your time and money.