Jugglez at Airsoftparts.ca is awesome.
There's another thread here somewhere (search tracer or tracer unit)...from not too long ago.
- don't have a longer gun
- you can choose to shoot tracers or not
- not as bright as a suppressor style since illuminated BBs sitting in the hopup waiting to be shot will start to fade.
- you may need more than one tracer mag
- makes your gun longer
- flashes with every shot...tracer or not
Personally, I think the suppressor style is better.