Its only beta, and I've done very few tests but there are a few gotchas of note for Prepress.
1. Generative Expand returns a DeviceRGB jpeg. So if you start with a profiled pic that does not match your InDesign Document's default RGB profile, there is a colour shift. You can put it right with Object>Image Color Settings. Bear in mind you no longer have the original image placed in your Indesign Document to compare against so should probably take a note of its icc before Expanding. If you start with cmyk this is not so quick or straightforward to fix.
2. Generative Expand returns the whole image regardless of InDesign's Frame cropping.
3. The Pixels that Generative Expand returns are not necessarily the same in the regions that had pixels to start with.
In my tests the long edge of the returned image was never more than 4000pixels. There is a warning that resolution may be different, but not obvious that the whole image gets downsampled.
Too early to tell but if you feed it an image where the final long edge is less than 2048pixels the results are really really good.
If you feed it an image bigger than that results for me in the expanded area were often pretty blurry/smudgy, and results in the non expanded area clearly worse than the original at 200%.
I say 200% (meaning Photoshop) because it might be that depending on the size of the print you still have enough resolution for print without noticing the downsample.
I'd rather see this feature in Acrobat than InDesign.
