There's a mid-'08-'09 season roster update, but nothing since then. I'm not sure that's even useful in franchise mode - I always start with the 2008 draft, which requires the default roster.
The NFL Head Coach game is pretty self-contained - there's no hooks to the NCAA games at all. Specifically, there's no draft class imports - the four stored draft paths are all the variation you get (or need, honestly).
As for what's great about it, look around the forum. I've been addicted to this game for at least eight months, and I expect I can play it for years and not bother with the new Maddens at all. The replay value is huge, especially if you prefer play-calling & strategy to stick-skills & twitch-gaming. Different strokes for different folks; this is my kind of game.