I've been using a R.A.T. 5 for a while, and it mostly fits criteria. It's not symmetrical at all, though. The weight is adjustable with the 5 and 7 series, and the length is also adjustable. You don't need to re-position your thumb to hit the forward/backward buttons at all, and there's even a side-scroll wheel above them that can also be programmed for use as back/forward (much easier to use this way, even). 5 and 7 are mostly the same, except 7 is more adjustable, and 6400 DPI twin laser vs 5600 DPI laser in the 5. I've found that properly adjusted, it fits your hand much better than any other mouse on the market.
http://www.cyborggaming.com/prod/rat5.htm
http://www.cyborggaming.com/prod/rat7.htm
PS: I agree, the original Xbox controller was the best. I have big hands, and the 360 controller is pretty freaking cramped.