NOLF would not be as remembered if it came out before HL.
Half Life was pure gold as far as SP goes, it was the last SP game i actually enjoyed. I remember paying $49.99 at walmart for HL with the intention of returning it after i had copied the serial number, mind you this was more than a year after its release.... i had downloaded the demo and after beating it i just had to buy it. Anyway, after playing the first week i knew i wouldn't be returning the game, i had to 'tip' the author, and i hadn't even played MP yet....
My first MP experience was great, i hadn't found the crossbow in the SP game yet but it was the first weapon i found when joining the game. It was awesome, i called my buddy that had been playing SP with me the night before to tell him about it and he came over immediatly (he still doesn't have a computer, 3 years later!)
I agree the alien levels were a let down, but it didn't ruin the game experience, i vividly remember actually jumping in my chair a couple times... those damn snarks (lights off while playing was a must btw).
I sure as hell got my $50 worth out of HL and i can't wait for a sequal. I could care less about TFC2, i just want to know when we are gonna see HL2.