I still wonder how anybody could NOT have a copy of HalLife by now. Been around since what, 1998? Had HL long before I even heard about ESF.
Oleic : $!0 for a classic game which has many great mods, ESF just being one of them, is a fantastic bargain. By buying through Steam (which often has great bargains, I think HL is actually quite cheap atm because of its 10th anniversary?) you can download HL to your machine at any time. It will always update itself. It will always be legit and functional.
You dont have to have a CD that gets scratched, or a serial that you must always have written down somewhere, or an installer that gets lost on your hard disk and collects viruses, because the game is always linked to your Steam account and yours forever. Unless you are using Steam, you will not be able to play mulitplayer at all.