Link says it all. I got the G602 for $45 to upgrade my Microsoft Wireless 5000 Mouse. Aside from the weights being the same and still wireless and capable of using my pair of Energizer Rechargeables, it's a bit bigger, has a nano-USB receiver (compared to the schlong my 5000 came with), has a few map-able buttons, an on/off switch (so I don't need to keep popping out my batteries), the same sensory tech as my 5000 (Darkfield Laser vs Bluetrack), a nifty storage slot for the USB receiver (compared to how the schlong clips underneath my 5000 in an ugly manner), more lights than my 5000 (which is a slight improvement), a switch that goes between Performance and Endurance on Battery Life...and manages the battery usage better.
If you want to invest a bit more, you can get the G700S for $50. It's got recharging capability, but is missing a bunch of the features above...but I think the G602 is the better deal.
Overall, I think if I endured using this old mouse for gaming...the G602's going to be a luxury .