The biggest problem with adding Lasers is size.....and being able to buy effective holsters. So, the guide rod principle is very attractive....adding no extra size to your weapon
BUT,
The guide rod lasers I have seen, don't match up, IMHO, to the abilities of Lasers like the LAS TAC II. Check this one out, I think you will be happy with the laser output ( brightness) and it's long service life on one set of batteries. The Laser Device Inc. laser I have installed on my P-99 .40 has a run time of 40 hours !! I have recoil tested my laser for over 3,000 rds with no ill effects and it HOLDS ZERO! I also demand that the laser dot be very well formed and "tight" out to target engagement distances.
JF