Has your friend used core.typed[0]? I haven't really, but I've wondered if it might be the perfect balance for people who really miss their type system.
On the other hand, Typed Clojure does not offer nearly the cohesive experience one gets from Haskell, nor do the types guide optimizations etc. Type checking is really a lint step rather than a build step. And almost all existing Clojure code is still dynamically typed, so you can't usually infer how to use a lib based on its type signatures (since it doesn't have them), which I love being able to do in hs.