Most are generally worth face value (or less), however, there are known varieties with colors missing and/or certain plate numbers that make certain examples worth quite a premium. I wouldn't classify them all as postage value only. You first need to do some research.
Your best option would be to identify what you have in the Booklet Pane Section of the Scott Specialized Catalog of US Stamps to get a feel for what booklets may be worth a premium. If you don't have a copy, many public libraries have them in their reference section.
As for value, keep in mind that the catalog value is only a guide and most examples sell for less -- much less -- unless an item is particularly rare or in high demand. Searching out sold listings in
ebay and other auction sites is usually a good indicator as to what the material really sells for on the secondary market.