It's not a pencil and a sheet of paper
A wacom tablet and paint.net?
"Essentially, all models are wrong, but some are useful" George_E._P._Box
It really depends what use you want for the model. If it's use to to let you understand the domain, or communicate to the guy next to you, a pen and paper are probably the best tool.
If you want to put it in a presentation, then Visio is good enough. ( For example, some of the OMG UML specs use Visio. )
If you want to perform more complicated checking and transformation ( unusual for a Domain Model ) or link it to a later implementation model, then a fully fledged model base UML tool.
IMHO, It's ill-designed.
Unfortunately, I haven't used a UML tool that was an easy to use and intuitive as I'd like ( the closest being OmniGraffle which had the ease of use for creating diagrams, but no UML model behind it ).