This is absolutely essential to my team and is one of the primary stops in preventing us from purchashing licenses for everyone. Klok makes it so easy to add time to your day (by dragging and dropping) that often users forget WHAT they actually did or WHY they did it. And, unfortunately, it is NOT very easy to add comments to a task that you just started. You must either expand your view so the new entry shows up large enough (not a good option) or hunt down the Time Entry for the item (an even worse option because of how much more complex the steps are).
In short, if adding time is going to be easy then telling Klock WHAT / WHY you are working on that task need to be just as easy. Otherwise, the user ends up with countless time slices that they cannot remember the specifics of.