UML and struggles with Visio

I've started working on a new project and were trying to design everything properly from the ground up. I've got a rough high level system architecture in place, and now I'm trying to do more detailed design of the individual components of the system. I haven't used formal UML much in the past, so I'm trying to force myself to learn and use it for my class documentation. After some rough sketches on paper and whiteboards. I have tried using Visio to create the diagrams. This has not gone well. Visio seems to force a very strict form of UML, if there is some sort of informal notes, or attributes I want to add, it is almost impossible.

Also it seems that everything you add to the diagrams has to be done through the properties dialog boxes. Why can't I just get a box with three sections, and type what I want. That would seem to be so much easier. Maybe the other method formalizes the UML (and maybe it could be used for generating classes?) but it seems like way to much overhead.

I guess I need to look into other ways to do this with Visio, or even look at some other tools for UML. Maybe a good open source tool?