Awesome game. Build fortresses, boats, submarines, airplanes, helicopters, balloon driven airships or all of the above in one vehicle using a completely free form physics voxel building thing.
All the details of your design come into play. Your vehicle will have a center of mass based on where you put your everything, a center of lift based on where you put your wings or hydrofoils, a center of thrust based on where your engines are, and even centers of positive or negative buoyancy based on where the lighter and heavier than water parts are. Having things off center applies torque to your vehicle which can cause it to flip or roll over.
There's fighting against AI in this game too. Weapons range from simple one block weapon systems including rams, old pre 1880s style cannons, machine guns and guided missiles. Watching cannon fire arc between opposing groups of ships and seeing blocks get smashed, hulls broken open, ships listing or sinking is great. Even better is when someone's ammo barrels get destroyed and there's a huge explosion with slow motion effects.
To help with the battling you can also design your own weapon systems such as cannons, missile launchers, lasers or drills that might be as large as one system spanning the entire size of your vehicle.
If that wasn't enough, you can fill your design with its own AI brain and various automatic control blocks, then tweak all the parameters so that you have your own AI controlled vehicles.
As for the cons, well there are plenty. The game is in a very rough state. There's a lot of content, but almost none of it is balanced very well or free of glitches.
I expect many of them to be improved or eliminated by development. The game has quite a learning curve. Some of that is due to complexity, which is good, but most of it is due to UI design which is both poor quality and very non standard. What other games use the t-o and g-l row of keys for controlling vehicle movement? It even varies from vehicle to vehicle based on weather you're using a ship's wheel, generic vehicle controller, or complex controller. Other games allow you to move a camera, vehicle and character around using the same 6-10 keys and a mouse or controller, so there's no reason why this one should be different. It's not like the system is any better either. Even after playing for 150 hours, I routinely find myself disoriented while switching between 1st person, 3rd person and build cameras, and pressing space to jump out of my seat when I mean to move the camera up, or vice versa. Or pressing w to move forward and being confused when nothing happened, because I'm in 1st person mode while seated and I meant to move the camera forward.
I've more or less given up on playing the campaign game at this point. It's an interesting idea and may get better with development, but right now it has a number of problems. Blockade ranges are the same for all vehicles, which makes weapons with a longer range somewhat useless, since they can't actually enter battle unless they're the closest... but if the closest spawn first, then the long range units can't spawn because they're not "in battle". I had an absurd situation in my campaign involving a fight near my starting fortress where I was able to toggle between playing a completely broken "force" near the enemy. My design had no weapons, no AI, nothing but hull pieces, and was nowhere near me.
Yet the enemy considered this a fleet that must be defeated before moving a few hundred meters closer to my starting fortress. There's a retreat button that allegedly gives up on a force and destroys it, but all this did was remove my fleet from play, allowing me to put my fortress in play... which would then proceed to shell the the enemy ships for a few seconds, because they would despawn. They're waiting to fight with the wrecked ship I just "retreated", you see. Player ships and fleets should probably be completely destroyed much more aggressively when the player or an AI mainframe isn't physically on them. It's kind of a mess overall at this point really, and could use a lot of work.
Luckily there's still the vehicle designer and single player story missions, which provides plenty of gameplay all on their own.
Lots of the physics values are suspect. For example, engine parts have a mass of 0.1, the same as a block of wood, which means they float. In general, the game is poorly balanced. There are a number of complex systems in the game right now such as engine design that have a few clearly efficient solutions rather than encouraging design tradeoffs for different sorts of things, and right now the best weapon system is simply lasers. The developer is aware of the last two problems though and will probably have them smoothed out by the end of next month.
Since this is an early access game, I'd like to offer some suggestions for improvements to the game:
I strongly suggest to the developers that they completely rehaul the control system and bindings to be based on the 9 keys in the wasd region, q-e, a-d, z-c, plus 1-3 other nearby keys such as shift, v, or r to get the total back up to or beyond 10. The f key could serve a similar role to what q does now except it would toggle between being at a control station and controlling the vehicle and moving your character around. Use alt as a modifier to toggle moving the camera. Now you can do everything you could do in the current control system, but it's significantly less disorienting, and also similar to how other games work.
I would also say the turret and gunnery system needs a complete rehaul. they look and control terribly, and it's far too easy to cause them to freak out or glitch. Motorized and elevation barrels shouldn't be a thing when there are already turrets that do the same thing, and moving barrels look terrible when they inevitably clip through other objects. Not to mention you can use them to phase into your own ship and shoot itself.
I suggest the turret system be changed as follows: get rid of moving barrels from custom cannons completely and make every turret either a horizontally locked azimuth only turret or a vertically locked elevation turret and allow them to be chained together exactly one time. The overall orientation of the turret assembly (for aiming purposes) could be determined by placing a specific block, or by letting a firing piece or similar block control its turret. This would allow the creation of much more sensible turrets that would actually aim their weapons without insane glitching.