It's a proper remake alright and Xen part is a complete reimagining of the original levels. Xen does drag on a bit longer than I'd like but I can't really hold that against the game.
I got way more story from CC's Xen, had a way clearer idea of what was actually happening and what history Black Mesa had with the place. It was definitely long, and the finale vs. Nihilanth was somehow even more confusing than the original, but I at least had a much better idea of what I was actually doing there.