G400 is almost the same as my MX518 and I can tell it's an excellent basic gaming mouse. The major difference seems to be doubled dpi. If my MX518 ceased to work right now, I might be tempted to get G400.
I have to note, though, that I haven't installed Logitech's software at all, and thus one of the extra buttons doesn't work for me. The two thumb buttons work naturally with generic Windows drivers (both in Windows and games supporting extra buttons). The dpi buttons are hardware controlled, so they work as well. My experience from years past is that Logitech is only capable of producing bloatware, which is why I never installed it. The hardware is good, though.