What you'll get is an opportunity to be among the first on and off the plane --
NW 757 have two sets of doors and depending on which one they use, you may not have any coach passengers walking by you during the boarding process, which means a bit of an opportunity to get something done.

You may or may not be offered something to drink during the pre-boarding process, you'll (as previously stated) get a more comfortable seat and may even get a foot rest, they'll offer to hang up your coat for you and during the flight you'll get something to drink in either a real glass or a real coffee cup and something like some pretzels or nuts.

There's also a bathroom that's used by only the FC passengers and some of the crew members (no standing in line).

Not exactly something I would spend extra money to get, but if they give it to me as an upgrade I won't complain.

Bob H