Began playing Black Mesa a few days ago. I played it once when it was free and was missing the Xen part. That was long ago enough for me to forget pretty much everything, or things did change much since then, I don’t know.
It is still a fun shooter and I really like the level design (I bet it got altered a bit for this version). You rarely have to run back and forth without new experiences. Sometimes you find hidden stashes to refill your suit shield and at the same time peek into an area you visit a bit later. So as ridiculous the whole facilities seem to be, they are not random and… well, believable ridiculous. 
Got to Xen last night and will get to that last part for the first time as I only played HL2 onwards.