Well, that was not my experience at all. For the most part I was able to shake him off by taking him for a spin over at the library or going through the East side of the building and going up or down the fire escape. In my experience, he leaves well alone after you have put a few doors/rooms/corridors between you and him. Getting rid of him is not that much of an annoyance unless you are doing a speedrun. There is then also the fact that he just sods off once you go into one of the "safe spaces", of which BTW there are a lot of. OTOH, there's the S.TA.R.S. office, the Clock Tower, the room with the licker coming out through the glass, plus pretty much every save room. There was only one time he seems to have stuck around, and I think that was because he was trying to get out through a barred door and just stood in front of it instead of heading for the other one (this happened in the area outside the Clock Tower), but I think that is a bug, or simply AI derp.
Talking about AI, his AI is derpy indeed and easy to game. In the library for example, I found out that when I went up the ladder and ran towards the stairs, the guy would follow my trail up the ladder instead of taking a straight line towards the end of the stairs. He would only take the 'smart' approach if I was way down the stairs and pretty much on his face. It is odd that his AI is so goofy, given that most enemies have solid AI. Phase 1 Birkin, in particular, surprised me by consistently trying to 'ambush' me by appearing on the opposite side of a piece of cover instead of just blindly following me. I guess the fact that X's behavior is 'emergent' probably makes it harder for the devs to get his AI to perform as well as in the more scripted boss fights. In any case, his AI might require some patching, but as of now, it is quite exploitable.
Furthermore, if you have your routes well-planned out, have escape plans, and you know the station's layout (which you should by the time X starts pursuing), it is not that hard to prevent him from catching you with your pants down, and the only situations where you are really in danger are licker areas, and at least two of those have a safe room nearby (the S.T.A.R.S. office in the case of one, and the photo room in that of the other). I only really had three "Oh shit!" moments with him, and two of those were scripted (when you grab the crank and getting out of the prison), and thus cannot really be blamed on his systemic behavior, which is what is being discussed here.