Lasers are great but how they are integrated into the weapon is often lacking...IMO the M&P (and possibly some other similar pistols) have it made...the laser replaces the changeable grips, leaving the user with a grip that is largely unchanged from stock. I've tried the Crimson trace grips for 1911 and M&P, and you barely know they are there.
Glock, on the other hand, doesn't offer suck options...so you're stuck with a MUCH larger grip profile, which on the already large Glock grip just doesn't do it for a lot of people.
I don't think I'd bother with most other laser offerings, either. Rail mounted isn't bad, assuming it has a TLR-1/X300 style activation. Small buttons SUCK, big levers are better. But with it so close to the muzzle carbon fouling can distort the laser leading to BAD things...
The lasers that replace the rear sight are downright retarded. I played with those just this past week, IMO completely unituitive. Very small button your have to press to turn the laser on, and then it's constant on. Press it again for strobe, then again for off. It just doesn't make sense, the button is in an awful spot, and should you need the gun RIGHT NOW you'll either spend too much time screwing with the laser, or you'll just not turn it on. Plus it makes for a super busy sight picture.
As far as low light, I think it's one area that most people really need to get some training. So many rumors on the internet in regards to lights, lasers and no/low light in general. It's a real eye opener.