Xen was great. It merely separated out those who got it and those who:
- didn't pay attention to the assisted jump training
- couldn't platform in FPS POV
- couldn't handle suitably odd and interesting environments vs normal real world stuff
It was a blast if you learnt the assisted jump properly and - for the time - a really unique setting with some very cool bosses.
Never had an issue with it at all and TBH was disappointed that it didn't feature in HL2 at all really.
Most Xen hate was probably fueled by the end boss, which is actually a major letdown as well as felt different from the majority of the game. Most other areas had a "boss" that had to be avoided or taken down through use of the environment. The end boss tho was a bunch of portal hopping and grenade chucking, which was both frustrating and anti-climactic.
The rest of Xen was fine.