Since the dawn of time I've used a Kensington ExpertMouse as my input device of choice. They've gone through some minor changes over the years but the basic design is the same: a "large comfortable ball" that is the size of a pool (billiard) ball that you control with your middle 3 fingers, and four buttons accessible via thumb and little finger. Coupled with the proper armrest, this is heaven. You don't have to pick it up to reposition it. You don't need a mousepad. Unlike trackballs that you control with your thumb, the Kensington EM being controlled by your index, middle, ring, or any combination thereof, provides an unmatched level of precise control.
Unfortunately the "majority" of the gaming market uses a mouse, so the concept of a "gaming trackball" has not made it past manufacturer's drawing boards. What defines a "gaming trackball?" It might be subjective, but to me it would be one that not only offers switchable DPI but the most important element: lots of accessible buttons. My Kensington with its 4 buttons and questionable programmability doesn't cut it.
Playing games that don't require a lot of fast-paced movement, you can get by with a trackball. Shooters and some of the newer MMORPGs however, require a level of control that makes using a trackball questionable. With that in mind, I set out to reluctantly add a mouse to my desk.
There are a ton of reviews on each of these mice, so I will keep my comments rather brief about each - and about what I like or dislike about them. I'd wholeheartedly advise reading more reviews and, most importantly, get your hands on one before spending money. Like keybindings, choosing a mouse is subjective and something simple like the shape of your hand or how you try to hold a mouse might sway you into a different model.
MadCatz Cyborg R.A.T Series
I picked up a R.A.T9 a couple of years ago for my FPS games. It is completely customizable and looks like a little Transformer on your desk. You can change its grips, adjust its spacing, and adjust its weight to perfectly work in your hand. Side buttons allow for DPS switching (sniper mode) so you can get fine-control when lining up a long range headshot then go back to speedy movement.
Pro: Customizing. Extra rechargeable battery to swap in (the 9 is wireless). Comfy. Unique looks.
Con: Not many buttons, but for Shooters this isn't a huge deal. Also, many complaints about the software and MadCatz products in general.
Would I buy it again? Before this weekend I would have said yes if I only played shooters. MMORPG, no. But after this weekend, just no.
Logitech 502 Proteus Core
I think this is Logitech's answer to the Cyborg RAT problem. It is somewhat customizable but not to the level of the RAT. Has the same three buttons on the thumb, one positioned perfectly for a sniper mode DPI switch. Has weight change. But overall it just feels a ton better than the RAT. The buttons feel more responsive and their placement is better. If I needed a few-button Shooter mouse, the Proteus Core would be on my desk right now, no doubt.
Looking at MMORPG mouse choices, the Naga seems to be on everyone's list, but I think that might only be because it was the first(?) to feature a panel of 12 buttons for your thumb. 12 friggin buttons. On your thumb. Support has been questionable, build quality has been questionable, the software to program the buttons has been questionable. I did not buy one though based on ergonomic comparisons.
Namely, the 12 buttons are on a smooth plane. This is good that you don't callous your thumb, but it's bad in figuring out which button you're about to press. The latter is the ultimate reason I passed this one over, but it might only be because I compared it to..
An obvious answer to the Naga, the G600 is Logitech's 12-thumb-button creation, only they used their brains a bit. As their current slogan goes, "Science Wins." The 12 buttons on the side of the G600 are arranged in two sets of 6, with each set being tilted inward to form kind of a valley for your thumb to rest in. Add to that a nub on one of the 6 buttons and you can most certainly tell what your thumb is about to press.
In addition, they added a third mouse button up top. Most all mice have two - left and right. G600 has a third for your ring finger. By default it is used as a "G-Shift" which allows you to program two sets of commands onto the buttons and "shift" between them with a simple click of your finger. That turns this "19 button mouse" (left + middle(x3) + right + 12 side buttons +2 under the scroller) into 38 buttons. They advertise it as 20/40 but one of them is being used as the shifter so they can't say that. Also the two under the scroller are kinda useless, IMO. Still, LOTS OF BUTTONS.
Pro: LOTS OF BUTTONS. More ergo for your thumb. Shifting = MORE BUTTONS. Ring finger button can be used for something else. Programmability of all Logitech products using their LUA-based software is wonderful. Did I mention the buttons? You could type on this thing if you really wanted to. The wired cord is braided.
Con: Overall ergonomics. Wired mouse.
I couldn't find a spot to place my thumb that made all 12 buttons accessible during combat. The natural resting place was inside the valley of one of the sets of 6, making the other valley harder to reach. Also, my ring finger wanted to be the right mouse button - something in my head said "click the RMB" and my ring finger did just that. I could have remapped things so that ring was the RMB and the ex-RMB was something else, but I found that I was also clicking it when moving the mouse around trying to push the thumb buttons.
Over time I imagine I could get used to it, but did I really need 40 buttons on a mouse? Would I remember where all 40 things were? Probably not.
Logitech G602 and 700S
Going to lump the G602 and G700S together because they are very similar. These are a more traditional ergonomic mouse design but with extra buttons. They are sculpted to be easy to hold, and give your thumb a place to rest. Each has multiple thumb buttons (NOT as many as the G600), each has multiple buttons next to the LMB, and each is wireless.
Thumb Buttons: 602 has 6, 700S has 4. The two extra are "nice" but not a dealmaker for me.. why later.
Extra top buttons next to LMB: 602 has 2, 700S has 3. The 602's are kinda flat and squishy. The 700S buttons are angled and raised enough to let you rest your finger on the middle one if you wish and easily click the top/bottom one from there.
Scroll Wheel: The 602 feels a little on the "cheap" side. The 700S has a nicer side-to-side click response (three-way button).
Wirelessness: As a bonus, the 700S has a (non-braided) USB charging cable for its rechargeable (replaceable too) AA battery, and while it is plugged in it disables wireless and acts as a wired mouse. So in a pinch if your battery dies, plug it in and keep going, just tethered.
Overall: Prefer the 700S. The buttons could feel a little more responsive (as it is they are kinda squishy too) but it works fine.
Corsair, SteelSeries, and a few others have gaming mouse offerings but as they did not have any demos at Best Buy, I can't offer any opinion on them.
There are some glitches with the Logitech products depending on what you do with them. There's a certain glitch that will cause your mouse to double-click instead of single-click if you try to customize it in a certain way. If you run into this, though, there's ways around it.
There's also ways of adding "G-Shift" abilities to any Logitech product. Want your 6 button mouse to have 30 functions in World of Warcraft? It's possible. Want to "G-shift" your mouse from your G13 gamepad or Gxxx keyboard? It's possible. This is why I ultimately picked a mouse with 9 usable buttons over one with 18.
The folks over on the Logitech forums work wonders through scripting advice and whatnot. And by folks I mean the users. The official Logitech people aren't much help. The aforementioned double-click bug has apparently been around since 2012 and they haven't done anything about it. if it weren't for the userbase coming up with their own fix, I would imagine Logi would be hurting.