Overall I think Xen was much better in Black Mesa than HL1, but why they felt the need to make it so long is a mystery to me. Half to two-thirds of the play time could have been cut and it would have only improved things.
It is so long, and I did not enjoy my time there. "Alright, here is the third or fourth energy door unlocking puzzle to slog through". It was so much slower paced you lose the thread from the human world of what you are even doing anymore.