Reasonably fun at the best of times, but unnecessarily frustrating at the worst. Its old low-poly charm is there and really carries my rating, but it's really hard to see why it's so beloved beyond that, there are so many parts where the controls, difficult enemies and quirky geometry detract from the experience with it dragging out towards the end with no goal.
I stopped before I got to Xen because of the horror stories from it, and since I didn't like it before then I'm keeping well away. Maybe my opinion will change down the line, but for now: a 2.5