Would be good to find out what reporting solution people are using and why. There are quite a few solution available at the app store such as simplechart, fusion charts, advanced reporting suite (only at appstore on mendix website) and built in solution. So far I've used simple chart simply because it is really quick to set up a graph without having to create a dataset/query/etc and no model required as it is a widget and not a module.
I prefer Simple Charts for exactly the same reason; it is easy to setup. Any necessary data manipulation can be done using microflows (or often I add a 'One' attribute, which just returns one, useful for aggregate functions like count). I have to admit though, I am not completely unbiased ;-).
The fusion charts module is especially useful when you want to configure your graphs at runtime, using the client.
The default reporting comes in handy when performance is an issue, and you are a skilled OQL writer. Furthermore it is integrated in the modeler which is an advantage to quickly see what you charts are about.