-You are able to see them in bright sunlight but they do jump out better in lower light conditions
-It helps when shooting from unorthodox/non standard shooting positions
-Adjustable for windage/elevation
-Great for dry fire practice
-They do have an 'on/off' switch so you can save your battery when practicing
-Crimson Trace provides replacement batteries each year for as long as you own the grips
-Dot size is 1/2'' at 50 feet
-Available for several size 1911 frames.
-Price. Generally the Master Series grips run $275 and up
-Can become a crutch; especially for new shooters.
Here's a video review with some shooting in bright sunlight, some night shooting, and a discussion of the pros/cons of the grips: