I know this thread has been dead for awhile, but thought I'd contribute. I'm brand new to this site.
I have a lot of hand/forearm pain issues, and have tried a lot of ergonomic mice. None of them are great for gaming, but...
First of all, most of those replying on this thread really didn't understand the nature of your question, as they kept suggesting "standard" mice and not truly "ergonomic" mice. Some mice are more ergonomic than others, but unless it truly allows your hand to rest at a 20 to 45 degree angle, they are not truly ergonomic.
The biggest drawback to most ergo mice for gaming is that in gaming you need to be able to easily pick the mouse up and re-position it. Very few ergo mice (especially those designed for a "palm" grip by resting your hand on the mouse and relaxing it, and moving with your arm instead of fingers) allow for this.
The second biggest drawback is often poor quality components. The makers of gaming mice seem to go for the really high quality innards, but fail in ergonomics. The most ergonomic mice are apparently made by "non-gamers" and folks who are way more concerned with ergonomics than quality of components.
The third drawback is the need to quickly and accurately move your hand from the keyboard to the mouse. Ergo mice often make this difficult. For office productivity, the need for slightly more careful hand positioning is not a problem. In the midst of a game, though, it can be life or death.
So, based on my personal experiences and trials (forgive me, most of these mouse trials were 2 or 3 years ago):
1. Evoluent vertical mouse (http://www.evoluent.com/vm4rw.htm
): very well made mouse, high quality parts, smooth operation, highest quality sensor in any "ergonomic" mouse on the market. Drawbacks: for gaming, hitting the buttons can/will move the mouse, so it sucks for aiming and gaming. Even hard in everyday "office" tasks when you click and the cursor moves from where you were aiming. The buttons can work with a very light touch, though, so if you get used to tapping very lightly, it can work OK. Also, it is so tall, that moving hand from keyboard to the mouse can cause you to catch your hand on the mouse, and knock it across the room. I was playing WoW while trying it out, and literally knocked the mouse across the room when switching from chat to mousing in a hurry. For my hands, I had a little trouble adjusting to the completely vertical grip, but a lot of people love it. There are more options now (including a small version), and I think it has evolved a bit now than when I tried it a few years ago.
2. Wowpen Joy mouse (http://www.amazon.com/Vertical-Ergonomic-Optical-Mouse-Ergoguys/dp/B001FWKA7A
) : probably the most comfortable hand position I have experienced in any ergo mouse. Truly comfortable design, but this mouse falls short in so many ways, it is unfortunate. First, it is so short that even folks like me with small hands find their fingers hanging over edges of the buttons. If you curl your fingers, and click with finger tips, it's fine. But ergo folks say you should be resting your fingers flat, and clicking with the lowest part of your finger (closest to the palm). Second, it feels cheap, and the buttons feel clunky and sticky. Third, the sensor is pathetic (800 dpi) compared to the 2500 dpi sensor in the Evoluent (don't even try comparing to true gaming mice). Folks used to higher end mice will hate it. If you are going to try modding an 'ergo' mouse with higher end sensor, etc., this would be a good starting mouse "body". Use some sugru to extend the buttons, put in a gaming grade sensor, and off you go. I ended up giving mine to a coworker with very small hands, and she has used it exclusively for 3 years now, I think.
3. Ortho mouse (http://www.amazon.com/Goldtouch-KOV-ORTHO-Ortho-Mouse/dp/B001QD8S86
): this one is very comfortable and lends itself to a full "palm" resting grip. I found, however, that the fact that the buttons are curved down so far cause me to click "against" my palm, often moving the mouse backward with the clicks. The customizeable mouse palm rests and such make if adjustable for a number of hand sizes and whether or not you want to rest your wrist on the desk surface or not. This mouse has a lot of potential, is very comfortable, and I used it for a very long time for office tasks. Also suffers from an 800 dpi sensor. Might be another candidate for a starting "body" to add better innards to.
4. Microsoft ergo mouse: was horrible. I bought it and gave it to a coworker after only a couple days trying. I am not a microsoft hardware fan anyway, but they completely missed the class on ergonomics 101 with this baby (at least for me).
5. A new one I have seen online but not tried is this one (http://www.ergonomics.co.uk/grip-mouse-right-hand-large.html
): can't find specs online, though, so afraid it's got cheap innards. It looks conducive to gripping and lifting for re-positioning, though.
I currently use a Logitech G700, slightly modified with Sugru (https://sugru.com/
) to make it easier to grip. See my pics. Notice I added a ridge on the right side to allow lifting the mouse (with ring finger) without having to grip it so hard (this is a heavy mouse, btw). On the thumb side I sacrificed the lower right (closest to the thumb) button to anchor more sugru to for a thumb side lift handle. In reality, the button is still fully functional, but I disable it. I added the soft pads to both sides as well, and I have no idea where the peel and stick rubber pad came from (I just had it lying around).
You can tell by my grip position that I try to maintain an angled "ergo" hand position, in spite of the fact the mouse really isn't designed for it. I like this mouse enough that I just live with it.
That is my 2 cents worth. I'd love to hear what the person who started this thread has ended up with.