I also have a G502 (mine is a Hero). I've had it for about 2 years and I'm pretty happy with it.
Upsides: I really like the little "sled" on the left for your thumb to ride on. I've never had this on any other mouse. Variable weight.
Only downside is that I sometimes bump the higher left side button on my kb when I'm not careful. This changes the DPI and screws me up pretty bad, esp. in games. Easy to undo with another button, but placement of that button is not ideal with my kb. More desk space could solve this.
Glad to hear its durable. Maybe I should buy another...
I think there's a "right" way to do this and a "temporary" way to do this.
Temporary: Paint with a brush and jam paint into the cracks to fill them. Paint again if necessary.
Right: Remove wallpaper. Paint. This will be MUCH more work and could lead to replacing the drywall which could uncover mold or other problems in the walls and turn "just painting" into a MAJOR PROJECT.
The choice is yours.