I was thinking this topic today, was annoyed with my current mouse scroll wheel middle button and started thinking what would be the optimal mouse for CAD. I noticed that there's been a lot of these gaming mouses lately, some of which have a bunch of extra buttons, and some even have advanced features like customisable palm rest position, etc.
I fiddled with a few in a store today, and noticed that the extra button placement is bit hit and miss: an extra button either is in natural position for you where it's easy to click with your finger, or it's not. This depends a lot how you like to hold your hand on the mouse. Even if it's "super ergo sculpted" with every button carefully placed, you need to keep your hand in very exact position on the mouse to reach every button, and this doesn't always work. I noticed I liked to have one small button top left of the left mouse button, so it's easy to click with the index finger, and also one button under the thumb. Usually, if a mouse had more than one extra button in for example thumb area, one of them would be natural to click, but others wouldn't.
Also second thing I noticed was the weight. Tried a Logitech G502 which was quite nice otherwise, but the weight at 150 grams felt a lot higher than with 'cheap' computer mouse that's well below 100 grams - up to twice as heavy! I think many 'advanced gaming mice' use metal frames, which bumps the weight up a bit. So I think my ideal CAD mouse would be something that has a small button in left top corner - which I seemed to like for replacing the middle button, would have some thumb buttons for extra functions, and has weight of 100 grams or less.
One model I'm interested right now is the new Mad Catz R.A.T. 4 - it's mid-tier in their gaming mice, which means it has plastic frame, but it seems to have all the functions (top left button, thumb buttons) I like and also has adjustable palm rest length. Note that their more expensive models like R.A.T. 8 have more adjustment options, including width, but these are metal-framed so more heavy.
**EDIT - the R.A.T. doesn't actually have a programmable top left button, at least with the current software, so something else might be better, like Logitech G402.
If you want a 'button monster', the mice oriented towards MMO games seem to have most buttons. However, the question with these is, can you actually locate the right button when you want it? Of course you could just map some buttons as 'no function' and add some sort of haptic feedback bumps to others. This is Logitech G600 for example, with 20 buttons:
There are also a lot of gaming keyboards, that add programmable buttons - these could be practical if you want to add a custom 'enter' key to the left side, or want some extra buttons between your 3DConnexion and the keyboard.
However, I'd still say that if you want a mouse that fits you perfect, you have to go to the store and try different ones out. Each mouse seems to have a different shape and button placement. I do know that some people love a certain mouse up to the point they bring one with them everywhere they go, but then again some people just use the cheapest one, so it's a very personal preference.