Unlike most of the other people in this thread, I love Island. In fact, it's the card with my highest +%. Is it that good? No - I like it too much. But it's really good for a big money strategy, and I play big money variants clearly more often and probably a touch better than most all of the rest of the top players (it's the action chains where I don't shine - I'm okay there, but not spectacular).
Objectively, Island is probably a decent to good 4, but not great. They're much worse in games with colony. It's often good to open with one, as you'll have very good chances of islanding an estate on the first reshuffle. And I love grabbing them early and in large numbers, though this isn't probably the best. Something to note is that if you win the island race 5-3, that's better than a duchy for the endgame. If you win it 7-1, that neutralizes a 5-3 province split.
But mostly you want Islands in the greening stage, particularly in a deck that builds a lead and then stalls out a bit. They actually go great with a card like chapel. You start going green much earlier than a normal Chapel/BM, and then when you can't make 8 anymore, grab an island. You can also island your chapel once it's dead. Don't think of them so much as a pure trasher - they're not any better tahn trade route for that, and trade route is horrible as a pure trasher. But they do help with your VP count in the end, and having this tiebreaker is pretty big.
So, overall, I'd say it's often good to buy one early, then grab them late, when greening. They work best in big money decks, or with a medium (steward, say) trasher or cycler (like warehouse). They don't work well with action chains, and don't combo with much but money. But they're pretty darn good there.
For a money deck, they're better than caravans, almost all the time. If you're playing for a combo, they're almost always gonna be worse, but even then you MIGHT (look at the board) pick ONE up in the opening and then go for them preferrable to duchies in the endgame, at least until late endgame when the one extra VP is critical.