Personally I'd prefer example.com/users/foo, or even example.com/u/foo, because (i) you don't need unusual characters like @ or ~, and (ii) you don't need server config to look for a wildcard in the root.

Alternatively, if as you say it is for hosting "user generated content aimed at technical folks", users might want to feel like they own their own (sub-)domain, so how about foo.example.com instead?