I have an Xbox 360 Controller for Windows myself, and I can tell you it's an amazing controller.
Pj64 fully supports it, of course, and it is as responsive as the real N64 controller was on the console itself.
Finally, you will find that the latest games have a very special support for this controller in concrete. For example, if you are playing Spiderman 3, you will see that all the controls have been automatically mapped to the same control scheme as the game on the Xbox 360 and, what is more, it even shows you the icons of the buttons on-screen as you play: what I mean is that in a tutorial where they teach you how to play, instead of telling you "press SPACE to jump" or things like that, it will say "press A to jump, B to punch, X to... etc" WITH THE PICTURE OF THE BUTTON on your controller. Just as if you were playing the game on the real 360.
I think that is a neat addition to games, and you will find that already a ton of them support this (rumble included).
As a plus, all the Games for Windows branded games like Bioshock, Tomb Raider: Anniversary Edition, Halo 2, Shadowrun and many more fully support this controller. In Bioshock for example, when you choose the control options, you have one saying "switch to Xbox 360 controls", and the whole game will change everything for you, like for example, in loading screens it will show you a picture of the Xbox 360 controller with all the actions the buttons do (if you have the Xbox 360 controls turned off, the loading screen will just show a keyboard picture).
So definitely I recommend buying this controller. It's really comfortable and responsive, and it has great support on the latest games (of course old games support it too). Buy it if you can.
Wow, it has been a long time since I last posted here, huh?