While controller appliances are still sold, there are a number of trends that are becoming popular, among these cloud based solutions and virtualized solutions. While this is a bit off topic, there are also enterprise vendors looking at implementing SDN concepts in wireless...but that is another discussion and we have yet to see how that will play out.
Additionally, vendors are pushing more of what has been controller based "intelligence" to the access point, including roaming and policy enforcement decisions. This is largely a factor due to concerns about the potential bandwidth needs necessary to tunnel all the traffic from 802.11ac access points back to the controller. While the implementation is specific to vendors, this typically takes the form of a "control connection" between the controller and AP, while the data is dropped off directly from the AP to the local network.
Even so, there will continue to be a need for some sort of "controller" functionality in some form. For example, something to act as the NAS for 802.1X authentications. It simply doesn't make sense to configure each access point as a RADIUS client on your RADIUS server to function as a NAS individually.
No matter how the industry develops, no good enterprise class wireless system will exist without a management platform that will manage configurations, provide AP updates, and collect data to provide reporting and troubleshooting, so these aspects of controllers will also remain in some form.
You mention Aruba, but they also have non-controller based solutions, namely their "Instant Access Point" solution. While a newer offering, this is an example of a move away from the controller appliance model.
Cisco has acquired the Meraki cloud based solution to add to their product line. Aerohive runs either in the cloud or as a virtual appliance (also sold as a hardware appliance if you like).
Ubiquiti's solution provides a low entry point financially, but as with many things this comes at a cost even if it isn't in capital. The software they provide does take care of a number of these important factors, but only to a degree as they are not as fully featured as other vendors products and doesn't have as many configuration options (which can be important in a deployment). It also isn't nearly as polished a solution.
It is not my intent to knock Ubiquiti, it is a decent product. I have deployed it at several client sites.
Ultimately, you need to look at the features and performance you need/want to find the solution that fits your needs and budget best. There are many wireless vendors and only you can decide what is "right" for your organization. I would recommend calling and talking to several vendors as they will be happy to demonstrate their features and even let you evaluate their products. Keep in mind that they will all "market" their product as the best, so you will have to do some independent research to figure out which of their carefully selected feature highlights are mostly smoke and mirrors.
