It was okay I guess?
They seemed way too small and mildly empty. It had nice NPCs but beyond aesthetics there was no reason to split all these across 7 islands.
4 Island in particular is awful. The only breeding center and an Ice Cave. Those could have gone anywhere
Island 2 is pretty much useless, even in post game (Island 1 at least had Ruby Path and Island 3 the Dunsparce area).
Areas like the "Water Labrynth" seemed a bit small for no real reason.
If they wanted to keep the island motif, ideally they probably could have condensed everything to about 4 larger islands. Keep 1-3 the same as they are Post-Blaine, but then open up the final island and more areas in the Post-Game.
While it was nice that a number of Johto Pokemon were available across the last 5 islands, I feel they probably could have added more. Maybe even throw a few of the really common Hoenn pokemon for variety (Zigzagoon, Tailow, things like that). I kind of wish they had put out-of-Kanto Pokemon into Islands 1-3, too; just imagine how neat it would be to grab a few early/evolved/single staged Johtos/Hoenns. "Whoah these aren't in the pokedex???" would act like a teaser and depending on the Pokemon could still be used.
It was really lame that beyond Ponyta & Magmar the first 3 islands were boring from an encounter perspective.
I did, however, admire the increased NPC interaction, some of the side areas, and the multi-island wide TR plot.