M2 Carbine got it right, those linked to are junk... useless... maybe for a classroom as a pointer... (er ahh, I would leave it off the gun for that one!)
Anything with Streamlights name on it will most likely be a good product.. they built for real world use, and there are dang few police officers in the country, and many overseas that do not use their products in some form..
Now a word about Crimson Trace...... Smokin... A numba 1.... I own about 6 or 7 of them, I bought them for my Family, and several of my friends... They are the benchmark for handgun lazer systems... Clean, comfortable, easy to install and maintain. Batteries are cheap and available at any drug/grocery/auto parts store... and the customer service is rocking.. (at least for me it has)..