I have the G7, and I've tried the G5. I've also owned an MX1000 and various other mice.
The G5 does seem to have a rougher surface on it than the G7. I prefer the smooth surface of the G7, but I could live with either. The G5 is nice in the fact that you can swap out the weights to really get the feel you like. It's ESPECIALLY nice if you're switching from a super-heavy mouse like the MX1000. However, with all the advances in cordless technology, I really don't see the point in getting a corded mouse anymore, unless you're running a cordless phone, many cellphones, microwave, or radar jammer right next to your mousepad.
I never really thought the extra DPI would do much for me, but I noticed a huge improvement from my last mouse (MX1000) as soon as I switched to the G7. Usually, switching mice will cause me to go into an adjustment period, when my gaming performance just... sucks. Once I get used to the weight, smoothness, etc. things go back to normal. With the G7, despite the switch from a heavier mouse to lighter, and different button configurations, I instantly noticed an improvement in my accuracy in BF2, CS:S, and even just clicking buttons or targets in WoW. I was floored to say the least. The only real use I've found for the DPI adjustment buttons are when sniping in FPS games - especially BF2. Zooming in with the scope and trying to hit that speck-of-a-person on a rooftop across the map is a lot easier if you can move your crosshair pixel by pixel, which lower DPI definately helps with.
Don't cheap out on a mousepad for either of these. They have very smooth, slick pads on the bottom that work best if you have a mousepad to couple with them. If you're using some generic junk, go out and look for something decent. Spend $20+ if you have to. I bought some light-up Thermaltake job from Radioshack for about $25. It's a lot more than I ever planned on spending, but it's been great to me. The surface is just rough enough for my taste, but most importantly, the pad doesn't move around on the desk at all.
Overall, I say go for it. If you're sold on wired, get the G5. Otherwise, give the G7 a try. Yes, the batteries DO die fast; however, you have two of them. By the time one dies, my other is charged and ready to be used. I've never been disappointed.