[samdk]

I'm really not the right person to be answering this. I haven't been at Jane Street that long, and I have no experience programming OCaml without Core. You'll likely get a much better answer on the mailing list: https://groups.google.com/forum/#!forum/ocaml-core. (I've also posted a link there to this discussion already, so hopefully someone will come by and give you a better answer.)

Probably the biggest difference between Core and the INRIA (default) standard library is that Core itself is mostly written in OCaml. This has many benefits: code is less verbose, easier to read, it's easier to verify correctness, etc.