Fry's Electronics out here in California has these and I got my hands on one today while I was there to pick up other stuff. I didn't have a surface to use one so I couldn't really gauge how heavy it actually was, but it didn't feel all that heavy to me. I'm sure it IS heavy as everyone's commenting, but it just wasn't so heavy that when I took it out I was thinking "damn this is one fat rodent!"
Anyway, I couldn't figure out how to work all the adjustments. This thing would definitely take some getting used to. It's a strange tactile experience because when you're holding it, you're holding rubberized surface in one area, touching plastic buttons with your thumb, and brushing against metal in another spot. The most distinctive quality of it in the couple of minutes I was messing with it was how crisp the buttons were. These did not feel like any normal mouse buttons that I've tried.
I'd love to try one of these in action, but just from messing around at the store I'd say I'm on the fence about it. It is just an all around odd tactile experience to say nothing about how it might perform.