At one point I was evaluating open source UML software for creating an overall diagram of our project to aid in understanding all the different moving parts at

At one point I was evaluating open source UML software for creating an overall diagram of our project to aid in understanding all the different moving parts at an overview level, but found most of them lacking. Especially in the area of generating UML from existing code. Does anyone have a suggestion for software which better fills the project comprehension/overview role?