Ha ha! lol, I'm using an MX400 and it's currently showing it's age. I loved the grip on that thing! . So to answer your question : I went to the store to look at the G500 and I realised it was bigger. Bigger since your thumb was supported by the big piece coming out of the mouse. I must say that I was skeptical at first but I came to like the feel of it. In general, I would say that the G500 has a different feel to it but it's a better one. There are a lot of features on it that can rival with 100$ mice. All in all, you can't go wrong with this one, if you liked the mx400 like I did, you'll love the G500.
In the end, it all comes to have you grip the mouse. If you liked the MX400 you are a palm gripper which means your palm touhes...