I really liked Xen chapters in Black Mesa. It felt like completely new adventure. And it nicely connected to the lore of Half Life 2 - the way they portrayed the Vortigaunts (they will be only hostile when under influence of the Controllers - you can finsih the Xen levels without killing any Vorts, in fact you will get an achievement for that, just take out the Controllers instead).
Also the fight with "ball-sack-spider" is epic and lasts for at least an hour while you play hide and seek with her/it.
I personally enjoyed it way more than original Xen levels, and to be honest it may be my new favority chapter now.