kawa125racer said:
It really depends on the make. Some of the high end CDI's (Takegawa, Tokyo Mods, and maybe a couple others) actually work by altering the ignition and you will notice a difference. I think the majority do very little other than letting you rev it a little higher.
I really like the Tokyo Mods. I've put them on a couple different bikes that had slight hiccups and the Tokyo cdi all but got rid of the hiccups. Granted, they are not a miracle fix all but will help with very slight tuning issues