It does not show correct pace when you are out running. Lets say you run 2 miles flat, the pace will vary on your watch, often with more than 20 seconds even though you are running at an even pace. Then, lets say you start running a long hill, and instead of immediately change to the pace you are running it gradually slows down even if you dont.
Why is it so critically acclaimed?
Also, if you use it at the track it will often miss with several meters making the pace completely wrong, and this on a flat track!
Third, often when used for interval, if you click the "lap" button it many times doesn't react at all. Extremely frustrating.