For future reference, it’s stickied at the top of this board.
Yes, it will work with any mod as long as that mod is running on the latest Steam build (i.e you need to own a copy of Half-Life on Steam for it to work).
It’s possible to play demos done on older Steam-based clients (like NGHL for example) in latest Steam by copying and pasting them into your steamapps/Half-Life/ folder and then using the ‘playdemo’ command, and if you want to use the ‘viewdemo’ command you can use HLAE’s demo conversion tool to make that work.