Tekken 3 works just about flawlessly, and with Pete's OpenGL plugin its absolutely beautiful, too.
Just like w/ N64 emulation (but even more so) the question of whether a game will play well in ePSXe is not a black and white thing. For example, in FFIX, every time there is a battle there is a swirling transition effect that takes you to the battle screen ... this transition effect runs very very slowly in ePSXe, but once the battle scene begins the framerate goes back to normal. It's tolerable. This happens even on relatively fast machines ... mine's a Epox 8KHA+, Athlon XP1700+, 512mb DDR, GF4 Ti4400.
Also, some games just plain don't work. Road Rash 3D is one that will not play. I was pretty disappointed about that. Tiger Woods 99 "plays" buts its not "playable."
I really enjoy the N64 emulation but I never got too much into PSX emulation ... one reason why is that you really need to copy the game to your harddrive to get optimal performance from your machine (do this with Isobuster). PSX games are pretty huge,so your harddrive free space will suffer. I've got about 14 games on my harddrive and they take up over 12GB (this includes some multidisk games like MGS and FFVII and IX). BTW, these are my own games. What you do is your business. If youre wondering why I would bother to play my own games in ePSXe rather than in the actual console, its because they look soooo much better and they load sooooo much faster in ePSXe that my PSX console has become nothing but a bookend now. (Except for when I get the urge to play some Road Rash)