Or are they a waste of money/gimmick?
Yes and no. Fancy laser sensors are. Weights may or may not be. Color LEDs are. Good sensors aren't. Good ergonomics aren't. Good buttons aren't.
Also, some non-gaming features are often bad for gaming, like tilt mouse wheels.
Some of the best gaming mice are at or under $50.
Logitech's G100s, G300, and G400s all have great optical sensors, that work well at high speeds on just about any surface, have good layouts, and plain scroll wheels; though the G100 had some jitter problems, IIRC, and I don't know if the s fixed it. If you're up for a lower DPI claw mouse, IMO, give the G300 a try. It's way more comfortable than it looks.
Steelseries and Cooler Master mice with the A3090 are also good to look at. Logitech's software is ahead of everyone else's, should you be concerned about mapping and macros and whatnot.