You can very quickly create online experiences. There is no programming that happens after the design of your experience, so you can avoid the hassle, time, and cost of working with a programmer. Ion is especially great for those low-budget projects that you just can't justify the additional production cost. You can also very easily provide interactive, dynamic designs through several built-in functions. For the most part, they're pretty intuitive.
Another really cool feature is you can set up "if this, then that" scenarios. You can set them up based on the the user's response to questions, to their geographic location, and more.
As a designer, sometimes it's very difficult to try and match an existing design. To a certain extent, you need to let go of the details, and follow the program's lead. You can obviously make lots of design choices, but sometimes the small details (like margins, or specific fonts, etc) can add a lot of time to the project, making it less cost-effective.
Although the built-in responsiveness is great, it's by no means perfect. You'll still need to go through each page and test it on mobile and other browsers.
Make sure you allow plenty of time to train on this product. It is very robust, but can be quite tedious to master. However, the live-chat helpdesk is very helpful. When comparing similar products, I would check to see what their support is like.
Our goal using Ion is to easily create interactive content without the expense of a developer. It costs a lot of money to develop content. In Ion, when we put content into an Ion experience we can quickly pull apart "snippits" and use them in additional experiences. As a result, the client saves time and money.