Ah...good thread for me as well.
I also got a EMS TopGun for Xmas and have been fiddling with it trying to get things to work well.
I've gotten the EMS driver Gun2Mouse to work, although it isn't the greatest. It also isn't all that stable under XP, and I have a very clean and trim XP installation.
I have a two trackball setup and those have never been a problem under Mame32 (v.116 I think). I was able to get the TopGun to work with Mame by unplugging one of the trackballs.
I tried the gun with both Operation Wolf and T2. All the buttons work just fine, but it was unusable to play the game. I tried setting the analog gains and digital speeds to the lowest setting, but the sight was just all over the place.
I briefly tried the GunCon2 driver, but my understanding of what to do with it is rather limited. I'll check out the aforementioned 45page thread for some pointers.
As many have complained, I'm not crazy about the distance I have to stand back from the screen. I even tried tilting the LED bars trying to move the convergence point a little closer, but no luck. At 6 feet from the 17" screen, it's not exactly a point and shoot game anymore.
I'll go back and try the GunCon2 driver again. If anyone can give me the 50-words-or-less explanation of how it works, I'd be very appreciative.