I played with recently at an event in Los Angeles. The DLC, which comes with the $25 Season Pass and will be separately purchasable for $12, will add a new 19 square kilometer island to the north, a new storyline and missions, and two mechs. While I didn't see the new island or any of the missions, I did get to pilot one of the mechs, using it to destroy a few bases, kill a bunch of soldiers, and see how it felt alongside Rico's other weapons of mass destruction. (Note: The gifs below were provided to me, and were not from my gameplay session.)
Adding mechs to the already ridiculous chaos of Just Cause 3 sounded like a surefire winner to me, but after about fifteen minutes of play I came away feeling disappointed. I guess I expected something a little different from Avalanche, who generally excel at creating gloriously fun tools of chaos and destruction. The new mechs, though occasionally amusing, felt like the weakest of Rico Rodriguez's arsenal of deadly toys.
When I think of mechs I think of giant, stomping, lumbering robotic suits, but these mechs are on wheels, so you drive them. The driving doesn't feel great. The mechs slide around as if their wheels are greased and the land is covered with ice, or like you're hovering a few inches above the ground. You slide around, basically, in a way that feels completely devoid of friction. I'm not asking for realism, not in the ludicrous sandbox of Just Cause 3, but zipping along in any direction at incredible speeds felt silly, and not in the good silly way that the Just Cause games often do. Maybe mechs that ran around on legs would have been too slow for JC3, but this greased lightning approach wound up feeling like someone just made a mod that turned off the driving physics. It's slippery and weird.
Picking up objects with the mech's gravity gun (called a gravity grip) and throwing them around is definitely useful. See a tank, or a vehicle, or soldiers, or pretty much any object that isn't nailed down, and you can lift it with your gravity grip, carry it around, and release it at high speeds, turning just about anything into a projectile weapon. I picked up objects and fired them into fuel tanks and radio towers, I picked up soldiers and threw them into each other, I carried one poor grunt from the base he was guarding, slid down a mountainside, sped across the map to the edge of a cliff, and launched him into the ocean.
I never thought I'd say this about ending a man's life by using a giant gravity-defying robot to propel him several hundred feet into the unforgiving sea, but the experience was joyless and empty. Again, it felt like a mod more than anything, like that mod for GTA 5 that gave you a gravity gun. Something to be played with for a few minutes—ha ha, I can pick things up and fling them around!—and then quickly discarded in favor of the more standard and satisfying ways of destroying things.
At least punching things with your mech's giant fist is fun. You can derail trains and wallop tanks and bash hapless soldiers and even leap into the air and uppercut a helicopter (theoretically, at least—I couldn't jump high enough to accomplish that myself, and seeing a helicopter directly above you while you're piloting a mech is difficult). You can also "ground-pound", delivering a superhero-like leap and and a landing punch to your enemies.
The few minutes I spent with the new mechs, though, felt like enough for me. It definitely didn't feel like something I'd want to use regularly in the main game. Keep in mind, I didn't get to see any of the actual mech-based missions on the new island—I was only turned loose on a few random bases—so my hope is there is content specifically designed for the mechs that will make them a bit more enjoyable.