I've got an mx518 also. Had that first, then got a G5 as a christmas gift, used it and liked it okay, but I missed having both the back and forward browser buttons on the mouse (which I could reprogram to various commands... i.e. back for vent talk, forward for squad chat was a common config). My G5 broke, and I went out and bought another mx518 cause I liked it so much.
The only problem I have with the 518, or at least mine, is that compared to the G5, the sensitivity controls are a lot more ... ambiguous, I guess? It's not clear whether or not it actually increased sensitivity or not, and there's no mini display showing what level it's at. Other than that, I love it!