I picked up a Logitech G700 on the recommendation/praise of many other users.
It is very comfortable, and there are lots of buttons. But there are a some glaring flaws in the design of the device:
First, you have to use Windows to adjust the mouse's settings, as Logitech (Still!) does not support using their gaming mice on the Mac OS. This has been brought up multiple times, but logitech's support forums are bursting with requests for a Mac version of the driver. After two years of requests, they have done nothing. So consider that before spending hard-earned cash to support them, because they sure won't lift a finger to help you when/if you have a problem with their product.
More glaring: due to some odd engineering decisions, many buttons on the mouse don't register as buttons in software. G8, G9, G10, and G11 are not available as generic, non-descript programmable buttons. By default, they act as switches for DPI and Profiles, and for checking battery life.
It gets better: you can't make these appear as mouse buttons, no matter how you configure it. I played with Logitech's software for over an hour; you can change these buttons to play back a macro or act as a key/key combination, but you can't make them plain old mouse buttons.
In the end, unless you use extra software (I use ControllerMate), you will really have a seven-button mouse with a few keyboard repeats that is advertised as a 13-button mouse. The side-scrolling buttons are also of dubious programmability.
In fact, the mouse will appear as a keyboard when you plug it in -- so don't be surprised when the "unidentified keyboard" window pops up.
Long story short, it can be a very nice mouse but it takes a bit of taming to be useful.