A great way to test if you know What Revit Wants is to try and run a complicated high rise or health facility using model groups. The principle and general functionality of groups is fine, but they can get very difficult to manage if not treated properly. However, they can be mastered.
As Ceilidh Higgins puts it:
Whilst groups are error prone and seem to have a lot of bugs … they are still the best available solution within revit for collecting together repetitive sets of objects.
She recently presented at RTC on this subject, and she has provided the associated presentation slides for download and viewing.
You may also be interested in this AU class by Aaron Maller:
Autodesk® Revit® Links, Groups, and Documentation: How to Make It Really Work!