What are the benefits of a gaming mouse?
Generally, weights and on the fly dpi changes make the biggest difference, but it's more of a subjective thing. If you're not looking to play competitively even on the casual level, then a gaming mouse isn't really needed, just get a comfortable mouse instead.
My favorite mouse right now is the Logitech G9, it's super comfortable in the hand, not too heavy and not gigantic.
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Switching from high-to-low (changing from speed to accuracy) in say a firefight can mean the difference between you getting a headshot and a bodyshot. This is why the Mad Catz R.A.T. is a great mouse, since it has a dedicated button in an easily accessible place to change DPI (the mouse itself has a lot of downsides though). It's all personal preference though. Crazy high DPI is unusable to most people and is just extremely frustrating to navigate with.
I hope that makes sense!
Also, I chose this mouse because it's extremely comfortable. A lot of generic mice are meant to be used with both hands, but this mouse is specifically for right handers, and makes it so I can play games for a long time without strain.
Also, I agree with the others, I've never had an uncomfortable gaming mouse. They all seem to be ergonomic. I especially love my Razer Deathadder.