I've played through HL1 3 or 4 times but playing Black Mesa I find myself wondering, "Is this a new area?" all the time. I'm really enjoying it.
I guess Steam doesn't track time played for mods but I'm probably over 5 hours and only just got "We've got hostiles." Mostly because of spamming the scientists in the intro and then quicksave/quickload like crazy during combat because "I lost 3 health?! I can do that better!"