There isn't much depth, but you'll enjoy yourself regardless.
You're probably aware that Reign of Fire for the Game Boy Advance is based upon the motion picture that also shares its name. Nonetheless, the game only loosely follows the plot of the film, which turns out to be one of its strongest traits since you're able to roam the postapocalyptic earth slaying dragons without worrying about pesky story details or silly quests. Another nice feature is that once you've played through 10 missions as the humans, you get to play six more as the dragons--and they're no less powerful when you're calling the shots.
To compare it to anything, Reign of Fire resembles classic arcade action games like Ikari Warriors or Heavy Barrel. For a more recent example, think of Expendable on the Dreamcast. Your viewpoint is above the action, and you're in control of as many as eight soldiers at any given time. Each of your allies has a rocket launcher with an endless supply of ammunition, and you all move and fire as a unit. When you move, they move, and when you shoot, they shoot. And the dragons have unlimited numbers, so the screen is constantly awash with rocket shells and exploding dragon bits. Throughout all this, your goal is to accomplish missions, such as trashing dinosaur eggs and gathering crops, although there are plenty of hard-boiled firefights as well. Ultimately, the dinosaur population is your main obstacle, because each of your troops can withstand being mauled or set ablaze only so many times. The mission ends when all of your soldiers die. In essence, it's a "quarter muncher" without the need for spare change. There isn't much depth, but you'll enjoy yourself regardless.
The lack of depth is the game's greatest flaw. Other than health boxes, there aren't any power-ups to collect that will improve your firepower or offer an interesting twist. In some areas, you can control crop carriers or drive around in attack jeeps, but these instances are too few and too far between to give you a real sense of omnipotence. If you want, you can earn better rankings by gathering all the boxes and barrels within each stage, but that gets old fast. To get the most mileage from Reign of Fire, you simply have to love shooting at things. That's not necessarily a complaint, however, since you'll eventually get to play as a dragon that can breathe fire, fly around, and generally slaughter countless humans without incurring so much as a scratch.
It's too bad that the graphics and audio don't really enhance the overall product. The color depth and liveliness evident in the backgrounds and characters are negligible, even if the cities seem realistic and the dragons look graceful. You'll enjoy mowing down dragons with the jeep's artillery cannon and observing how the screen can simply fill up with rockets and napalm, but you won't be impressed in either event. So too, it's nice to hear the cries of dying dragons and mutilated humans, but the plain old bullets and generic drama music only go so far.
As it is, Reign of Fire is really fun and truly invigorating--at least until you play through all 16 levels or grow tired of killing things. It's enjoyable enough to revisit later on, but even so, it's somewhat limited.