Consideration number one is, of course, matching the size of the case to that of your mixer itself. That doesn't necessarily mean you need a custom fit - for example, cases like the Gator G-MIX ATA Deluxe Rolling Mixer or Equipment Case are multi-purpose designs that can accommodate hardware within a certain range of sizes. But if you do value a totally customized fit, you can find that as well from mixer-specific enclosures such as the SKB Watertight PreSonus Studiolive 16.0.2 Mixer Case, designed to provide tailor-made support.
There are features that separate one mixer case from another beyond fit alone, of course. For instance, some include built-in wheels while others may not. Some have plastic shells, and some are road cases. There are even soft-walled models like the Gator G-MIX-L Lightweight Mixer or Equipment Case, which cuts down on weight to make transportation easier and more comfortable. And there are cases with added storage space, including the Gator GPA-720 Rolling Road Case For Powered Mixer, that allow you to take along all your other gear together with your board.
There's no talking-up required to justify adding one of these enclosures to your kit: the bottom line is that if you own a mixer, then mixer cases are for you. The only questions you need to ask are which ones will fit your mixer (or mixers), and which features you'd like to see in your eventual choice. Asking those two questions will help you narrow down your options until you emerge with a clear winner to put on your list.