Em gun games are awesome. There were a couple of different styles made - The light sensor type (light from the gun would shine on the target sensor), and the "dale gun" style, where there is a stylus style contact board mounted under the gun/rifle that moves with the gun, and touches contacts to allow the machine to know if the target is lined up or not.
I own a dale gun game (Chicago Coin Twin Rifle) myself. From what I have read this particular game is easier to fix if there is an issue with the guns - the guns are easier to replace since they do not project light, and I also remember reading that the sensitivity of the light sensors can change over time, making the light gun games more difficult to repair.
Most games are the dale style, and are more common, but I would love to own several of the light gun games if they were complete (some of the coolest in my opinion).
You can check out all the different models ands types on pinrepair (as already noted here by another post), to see which types use which target method.
If you can pick one up, I say go for it. For me, the 2 player games are the most fun though.