The main thing to understand about Black Mesa before buying it is that it's not a new Half Life game, it's a full-conversion mod to remake Half Life 1 in an engine already 15 years old. It's dated, it's clunky and has all the drawbacks of 90s gaming. That being said, I'm really glad I played it - but you can't treat it like a modern game. Black Mesa makes Half Life look like how we remember it from our childhood, rather than how it actually looks - which I think is worth paying for as long as you're willing to take all it's minor annoyances along with it. All this time later, it still feels unique and has some amazing story-telling and set pieces built into a very memorable puzzle FPS.
Playing Black Mesa has made me think about gaming history again too; although I do find it rather strange that Half Life 1 (1999) has been re-made in the Source engine (2004) when it was only released 5 years after the original. The result is that a game being released this year already looks dated. The answer obviously is extra money for valve and an unusually long development process (10 years). Still - I do think it's great that we can play Half Life more or less how our brains remember it, and not how it actually looks, which is pretty terrible by modern standards.
These are mostly standard to that era of gaming and not really specific to Half Life.