The best thing about modelio is its ability to help in creating diagrams for different purposes like business, technical or even educational purpose.
I haven't found anything bad so far but it's always better if you find something wrong with any product before using them as they can change their features anytime later. Its free version allows only basic diagram creation which makes me think twice while choosing this application over other applications available out there. It helps us create models of our projects very easily without having much knowledge of programming languages used by these programs.