<GASP>
I use the biggest LL Bean daypack/bookpacks as BOBs, FAKs, etc. They blend in easily in most uses, they're fairly rugged (I've been beating them for years), and don't cost as much as decent replacements. For the mil-look crowd, you won't like them. But they do blend in well...as do most bookpacks. But most bookpacks aren't made very well. Bookpacks have the advantage over most daypacks in that they do have some attention to accessory pockets and sleeves. They don't act like your molle gear though.
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...got YAK???