Seconded on all the MX518/G5 recommendations; I've used tons of mice and always keep coming back to these ones. I prefer G5 to MX518, but if you go the G5 route, you should probably be aware that certain reflective surfaces (Icemat, etc.) may cause tracking problems, although the newest drivers may have fixed this.
G5 ftw. I'll second the durability comment; been using mine for 3 years and it still works/looks brand new. The easy sensitivity adjustment is godly.