One of the issues in the past with updating Apple devices to the latest version of iOS has been the size of the update itself. iOS 7 required 3GB of free space and iOS 8 required around 4.6 GB. Being that many people decide to buy the cheapest iPhone or iPad with the least amount of storage many times having this much free space was nearly impossible.
iOS 9 is going to only need 1.3GB. This is still a large download, but most Apple devices should be able to handle it.