Working all day with screen, keyboard and mouse is quite a stress to some parts of your body. Over the years, getting old, you have to pay the toll. Sore wrists and tired eyes can become side-effects of your hard work. So far I'm surviving quite well but I do pay a lot of attention to anything which helps.
I'm addicted to Microsoft hardware. Their gull-wing keyboard is awesome and their mice are quite comfortable as well. I don't believe there is one mouse model which is the solution to RSI prevention; what really helps is switching continuously from one model to the other. So I have a couple hooked up together with the keyboard in a little goodie bag USB (1.1 is good enough) hub.
A "normal" mouse a monster trackball and just recently a wireless notebook laser mouse joined the club.
The latter has one, thumb operated, magnify button on the side. Which fires up the Intellipoint (MS mouse software) magnifying glass. Which is another big help. It can be just the tool to find that little pesky typo in a query.
The magnifier can be fired up with another mouse as well. In the configuration dialog you assign the function to a button. It took me my new mouse to find that out. In Vista the magnifier is part of the OS itself.
There is one thing which can be improved in this scenario. The mouse configuration lets you select one type of mouse and its button configuration is used for all mice attached. Enabling the magnify button on the laser-mouse turns the forward button of the other mice attached into a magnify button as well. I just wish I could configure the three mice independently. Beside the buttons it would make a lot of sense to set precision and acceleration different for each model. Something to check in Vista.