We have sailed in both the Junior Suites and in Cat Ds and Es, which are the balcony staterooms.
The most major difference between the JS
and the others is, as the Lori above mentioned, the double cruise credit earned in a Junior Suite; this would matter to RCI fans who are planning on many more cruises with this line.
The second most significant difference is in the bathroom: JS
has a bathtub with shower; the others have a small bathroom with shower only. We are both very large people, but we survived the small bathroom quite nicely.
The balcony, closets and other storage spaces, and the room in general will be largest in a Junior Suite.
Although we love the extra space of a Junior Suite, we found our Cat D and Cat E cruises to be every bit as enjoyable overall.
One little tidbit about the smaller rooms, which I like, is that the beds are close to the sliding glass doors/balcony. I can lie in bed and look right out at the ocean. In the Junior Suites, the "living/sitting" area is closest to the door/windows.