Razer Naga 2014 http://www.amazon.com/gp/product/B00E8CF268/ref=oh_details_o07_s00_i00?ie=UTF8&psc=1
Probably one of the best mice I've ever owned. The keys on the side are mechanical and responsive and map to your number keys by default, or you can set them to macros through the software for your favorite MOBAs or MMOs, if you decide to use them.
It also goes up to 8200 DPI, which is great for me because I actually have four 23" monitors to span horizontally and vertically (large screen space). And, I only keep it at 7000 DPI.
The back/forward buttons are on the top of the mouse below the scroll wheel. For aesthetics, it glows green, if you care. I have smaller hands and find the mouse comfortable to use for hours without any problems. I've owned it for about 6 months, and it's taken a beating, so it's quality made.
Anyways, hope this helps.
I use the Razer Naga 2014. The mechanical side buttons feel really nice to use. Razer's "Synapse" software works decently well with Mac, as you can bind any button to a multitude of functions. The only issue I've run into is that the OS-level mouse tracking seems to reset to 0 on reboot. Not a huge issue. I would definitely purchase this mouse again if mine were to ever go up (had it for about 11 months now with no issues.)
The Logitech G600 has decent reviews, although I didn't go with it because a) the side buttons are squishy, and b) the software only works in Windows, so if you want to rebind keys to something else, you're out of luck.
A cheaper option is the Redragon Perdition, which is a newer "MMO mouse." I couldn't verify, but I don't believe this one has any Mac software either. However, the buttons are still just numeric keys, so if you don't need anything crazy, it should be fine. You can also store the button profiles on the device, so if you have a Windows install, you can apply it there then use it in OS X.
There's also the UtechSmart Venus. Not bad reviews; fairly new; cheap. Like the two previous mice, I don't believe it has any Mac software, but the extra buttons will still function.