Depends on the handgun, I'd say, as there are multiple designs from multiple manufacturers.
I've got Crimson Trace grips on my S&W 642, and they're quite an improvement over the mediocre and non-replaceable sights. For close-up individual work, they're hard to beat, and even if they fail, you're no worse off than with regular sights. They're also very good for training, especially with DAO revolvers like the 642, as it's obvious if you're having problems controlling the gun during the trigger squeeze.
At a distance, it can be much harder to find the dot, particularly if it's bright out, and if there are multiple people using them, it's very easy to get confused about which dot belongs to which user.
Even with the CT grips, there are different grip materials, different switch configurations, etc. Some semi-auto designs replace the guide rod, which worries some users, as a mechanical failure in the replacement part can disable the gun. I don't have any experience with these, though.