I've done this also. I built a little box using an IPAC/OptiPAC, ps2 connectors
and dsub-9 and 25 connectors. My hope is to interface all kinds of old controllers to it. Right now all I've had time to do is the 2600 controllers. But boy does it work great. I played some Archon and One on One the other night for the first time w/ a second player and it worked perfectly!

Combat and Space Invaders are so much more fun with these original controllers. It's also very easy to use with emulators that do not support key remapping by using the programming utilities for the IPAC.
I started working on the Atari paddles but they use a POT and I wasn't sure how to hook that up to the Optipac. So what I started doing instead was using a hacked mouse inside of the paddles. Anyone have any tips on how I might be able to do this without so much custom work being done to the paddle?
If anyone is interested in pictures of this contraption I made I can get some to put up here.