I bought Advance Wars: Days of Ruin the day before yesterday, and I have been having a lot of fun with it. While I haven't played quite enough of the game to justify talking about the story yet, I have had a chance to try out all of the units in the game, so I will discuss that.
The units in Days of Ruin have changed a lot compared to the previous three entries in the series. There are a lot of changes to unit costs, many units introduced in later games have been removed, and a lot of new units have been added. For now, I think I will focus on the newer units.
Bikes: The new motorbike infantry units are a surprising change. In the previous game, the main benefit of using regular infantry over mech infantry was their improved movement. Now, bikes are the ones with great movement, so normal infantry do not have as well clarified as a role. Also, using transports to move infantry to cities is not as mandatory as it once was. I think the pros and cons of this new unit balance out to make the early phases of a map a bit more varied, though, so I think it was a good addition.
Rigs: Since Bkes steal half the purpose of the original APC, that unit has been redesigned as a more general support unit. It is still good for transporting infantry, though now it is mainly used for getting infantry across rough wasteland or forest terrain that bikes can't cross quickly. The interesting new trick is that Rigs can build temporary Seaports and Airfields, which makes it a lot easier to resupply air and sea units, and is a great innovation.
War Tanks: These things are the replacements to the Megatanks and Neotanks of Advance Wars: Duel Strike, and I think they work a lot better in the game. The old Megatanks were simply too slow and expensive to ever be useful, and Neotanks were simply too fast and powerful for a ground unit, especially with their relatively low cost. Wartanks though, manage to strike a balance of being strong, but not overpoweringly so, and slow, but not unusable. I am not certain that I would ever want to build one myself (I mostly stick to normal tanks, artillery, and air units), but they make interesting opponents in the game, so I will call this one an improvement.
Anti-tanks: This weapon is an odd one, being an expensive artillery unit that doesn't have a minimum range. I really don't like this unit. It is a bit too fragile for its ability to counterattack to really come into play, and it doesn't really have that much more power than a normal artillery unit. It seems hard to justify its very high cost (nearly twice that of a normal artillery unit). I like the idea of adding more to the artillery units, but I find this one to be forgettable at best. However, I admit that I might be underrating the fact that it is an effective counter against all types of tank, and that there are no direct-attack land units that are really effective at destroying one.
Dusters: These planes are a great addition to the game simply because they add a much-needed middle tier to the Advance Wars aerial units. Before, there was nothing between the cheap and low-powered helicopter units and the very expensive and powerful Fighters and Bombers. Dusters fill that role, letting you build a great aerial anti-helicopter unit without resorting to expensive specialized Fighters, so they work very well in supporting ground forces and helicopter units in the mid parts of the game.
Gunboats: These boats are a somewhat more combat oriented replacement to Duel Strike's odd Black Boats (which were made somewhat obsolete by Temporary Seaports). They make it cheaper to move infantry units between islands, and they are a much cheaper alternative to building Battleships in order to fight Cruisers. They are a bit difficult to use (I think they just move a bit too slow), but they are a good unit.
The new Battleship: Letting a Battleship make an indirect attack after moving was a great improvement. I never found Battleships to be very useful in previous Advance Wars games, but they are very useful in this game.
Carriers and Seaplanes: This is another old unit that has been changed so much it might as well be new. Now Carriers, which were only of dubious use in Duel Strike, have been improved so that they can build Seaplanes, extremely versatile flying units that are only slightly more expensive than Dusters and can attack any target, but have much more limited fuel and ammo than normal planes. I like this design very well, but there is one major problem: Seaplanes are just too strong. A Carrier would be a great "ultimate" unit even if the Seaplanes were not so powerful, but as it is they tend to be somewhat unbalancing. Even when I blundered and let a Carrier get quickly destroyed by an enemy Submarine, I still was able to devastate an opponent's main attack force with just two Seaplanes.
Overall, I like the way that the designers of the game focused much more on adding to the low-tier and mid-tier units in this game. Earlier games fin the Advance Wars series fell into the trap of constantly adding on more expensive and powerful units to create new top tiers, but that is not good game design. In a game like Advance Wars, most of the game is spent using larger numbers of weaker units, and many maps won't even support the use of unnecessary expensive units. It is better to make add to the variation of the units that will actually be used on a regular basis, so there are more available strategies.