Yea, it'll work on a PS1.
I love the controller; it's completely replaced my official Sony DualShock. (And I do *not* use 3rd party controllers.)
The sticks are super tight, and the buttons are all pretty good. (The D pad is a little rough to use in fighting games, so I can't recommend that.)
The shoulder buttons are really close together, and you have to completely relearn how to use them. In a few hours, you'll be just as fast as w/ a regular DShock controller.
The design is small, almost the exact same size as an original NES controller. However, it works great for me, even w/ my large hands. (Total NES nistalga. Yippie!)
I suggest the controller if you still have your old NES hooked up (and actually play it). It's a great controller, and once you get used to the shoulder buttons, will own you.