I like that you can take screenshots of websites, applications or even individual windows/panes with it's many features (and customize them).
The interface isn't exactly intuitive but in time one gets used to how everything works so its not too bad at all! It doesn't have as much options compared to other similar products out there such as Snagit which has more editing tools available etc., however this does come down mostly if your just taking simple screen shots without any edits required (which most people would be doing anyways!). Great product overall though very easy once learned. We use greenshots mainly when we're trying to show someone something online they didn't see themselves where possible errors exist within an application/website being viewed by another person who knows nothing about what was going wrong beforehand.