I don't believe I ever saw Mirage Island personally, whether on my game, my brothers' games, or a friend's. The concept of the island was interesting, but it was just frustrating to check every day to see if the old man could see it. But outside of the Liechi Berry, I suppose it doesn't hurt to have missed anything, especially since my battery dried out a few months ago (took it long enough).
Edit: Actually, according to Bulbapedia, Mirage Island is not affected by the battery, it's only after the play-time's been maxed out that it stops. So I may technically still have a chance...