MX518 i had that mouse in years... and very good grip on it, i tell you Logitech has the best quality! If Logitech, Microsoft and SteelSeries made a mouse together. The Laser from Microsoft Intellimouse Explorer 3.0 and the quality from Logitech and the coating and drivers from SteelSeries... it would be the best mouse ever.
I used MX518 for about 3 years, G5 for about 3 years, G500 for about two years and having been using the G400 for about a week now.
G400 is definitely the best mouse I've ever tried. You cannot go wrong with it, especially if you liked the MX518 (which it is a remake of). I'm only slightly lukewarm towards the software. I'm quite used to Logitech Setpoint but the G400 uses some new Logitech Gaming Software. Not sure what I think of it yet, or as to why the change was necessary.
In my opinion - yes, but they do feel very similar. G400's shape is a little more ergonomic, I believe. I unfortunately cannot test because I was using the G9x while I 'transitioned' between the two mice and I no longer have the G500 around.