Having spent several hours at Jewel on Monday afternoon, I can echo many of the comments above.
The center of Jewel is devoted to the waterfall and the forest around it, and both are definitely worth seeing. The heat index outside was 106, so being inside by the waterfall felt great. In the evening the waterfall was lit in a way that made it resemble the supertrees at night.
The perimeter of Jewel contains 5 floors devoted to shopping and eating. Much of what you see there is generic mall type stuff, but you can find some interesting things if you look. On the first floor, for example, I found a store specializing in interesting Singapore desserts, including duryan. They were small, tasty and fun. The very top floor is also quite pretty with something resembling "indoor al fresco" having the feel of nice sidewalk cafes even though you are indoors. The Tiger Lab was a very pleasant place to sit with a beer and decent noodles.
For a quick bite, going to a hawker center might be more fun, but if you have a long layover at Changi, Jewel is worth a try.