I've used both the Blinder Extreme M20 + Laser Interceptor. Both had two forward facing receivers/emitters. Both were installed on previous cars (STi's) - I haven't yet ordered a jammer for the S4, but plan on doing so soon.
I've had very positive results with the Blinder. On more than one occasion it has alerted me to (and presumably jammed) laser. When it activates, I typically check my speed, quickly adjust if necessary, then switch off the jammer. I've noticed many confused faces when I then pass the guy shooting me.
I had the Interceptor on my most recent STi, but sold it before it proved its value. So I can't comment on the effectiveness of that unit. I bought it due to the high marks from http://guysoflidar.com/
- so I presume it's quite effective.
If I do install a jammer on the S4, I'll make sure to post a DIY with lots of pictures. =)
2010 Audi S4 - Ibis White, Black Alcantara, S-Tronic, B&O, Rear Sport Diff
07 Impreza WRX STi, SPT Intake + Exhaust
04 Impreza WRX STi