Useful for training. Limited usefulness in certain self defence situations. But, not a replacement for marksmanship. Never trust your life to something with batteries, and circuits! The problem with new shooters is if they learn to rely only on the laser they are inefective if it doesn't work, and may waste valuable time trying to get it to work.
Great aid in learning instinct shooting though.
Cheapshooter's rules of gun ownership #1: NEVER SELL OR TRADE ANYTHING!