In my opinion Visio is a little bit too anal-retentive when it comes to UML. One of the first things I download after repaving a machine is therefore Pavel Hruby's collection of guaranteed 100% interpretation free Visio UML stencils.

When using these shapes, Visio doesn't try to understand or verify your UML diagrams but allows you to just draw. Adding properties to a class for example does not mandate crawling through a four-layer-dialog but can instead be done by just typing in the corresponding part of the shape.

Great if you use Visio just as a documentation tool and not as a code generator. And you really wouldn't use Visio as a codegen, would you?