UT99 (or UT-GOTY, whatever you want to call it) is still one of the best FPS to date, though it has several compatibility issues in modern systems (both software and hardware related).
Namely, the game runs either way too fast or too slow (making it unplayable) and it also looks very dark (no matter what renderer you're using).
The first important fix improves graphics and solves the darkness issue by adding new renderers (both D3D and OpenGL):
http://www.cwdohnal.com/utglr/
The second important fix solves the insane movement issue, making the game playable:
http://coding.hanfling.de/launch/
Hope somebody finds this post useful