It's the apocalypse, things went bad, and it so happens that you are the Overseer of life giving Vault-tech technology. People are literally lining up to get in on this deal. Sound interesting? 
In Fallout Shelter, you play the role of a Vault Overseer - responsible for dictating the every move of a Vault full of infinitely obedient Dwellers as they seek to survive, repopulate, and possibly even find happiness. Bethesda's new mobile tower builder was announced with a good deal of fanfare at E3 in conjunction with news about the upcoming Fallout 4. The game is initially iOS exclusive, but will come to Android in time and it is already wildly successful - topping the likes of Candy Crush Saga on the revenue charts temporarily.
Upside-down Tower Building
The core gameplay of Fallout Shelter is very akin to mobile tower building pioneers like Tiny Tower: make money, recruit Dwellers, build bigger Vault. With this overall arc comes plenty of opportunity for micromanagement, and the game does an excellent job of sucking up any free time you have to tend your little zen garden whether it be three minutes or thirty. The balance is superb for a mobile game, giving you enough to do with fairly well designed touch interactions that you really feel like an Overseer - dictating the actions of every Dweller in your Vault. The over the top 50's style cartoonish art style also contributes an interesting layer to the experience - sometimes making your Vault an idealized caricature of the harsh realities of post-apocalyptic life.
Climbing to Upper Micromanagement
On top of the fact that Fallout Shelter is fun, it stands out to me as a free to play game that gives you a ton of things to do without being in your face about making in app purchases (IAPs). In fact, I played the game for hours before I even figured out how to purchase the premium IAP currency - lunchboxes. You really don't need to purchase lunchboxes to enjoy the game, but as you cultivate your Vault you may find that you want more unique Dwellers, Outfits, or Weapons - and these are all things that you can fast track by purchasing a lunchbox. Overall, the model makes me hopeful that this is the first in a series of mobile games that can figure out how to offer a quality free base product with a monetization model that isn't intrusive or coercive. Hats off to Bethesda for a bold first step in that direction.
Radroaches and Other Bugs
Despite so many highlights, the game does suffer from a few series bugs that are large enough to unfortunately completely tarnish the experience for some players. There is a "sleep mode" resource drain bug that causes the Vaults of some Overseers to decay to a destitute state while they are away, which makes returning to their Vault a huge uphill battle to recover to a normal balance of resources. Early in the game before much gear is available, there is also a balance issue with Radroaches in big rooms that can result in a Dweller death domino effect (3DE, if you will) which can be frustrating and in some cases Vault ending if the financial impact can't be absorbed. I'd like to think that these issues are simple growing pains of the initial launch that a solid developer like Bethesda will work out in a future patch, but as it stands they do have the capacity to sometimes taint an otherwise amazing game experience. If you are experiencing these issues, check out my other post on tips for playing Fallout Shelter which has some strategies for working around these issues.