So, I just finished the game... or not! I beat the Boss, went through the portal, left the island on the awesome airship, and all together it was awesome. But I had to "spare" power gems, and there was that locked portal down on the first level of the castle (I assume the boss dropped the keys to that, and I just missed them). So there is more. After checking online, I know that there is, in fact, a lot more.

But is it worth it, should I go back, and keep playing? I had great fun with the game, a real blast, but I have that "my job is done" feeling. I escaped the Island, which was my goal from the get go, and I feel like I've dungeon (and forest, and swamp) crawled enough. Is the "true ending" worth busting my ass? Cause honestly, I felt fine just getting the hell ouf of Dodge...

Since you've fairly close to it, I'd say it's worth going all the way, if only so you know "how things could have been".

I've just completed the true ending and although its quite hard and frustrating I think the ending explains a lot about the nature of the island.

Yes it is worth it :)

Not only does it explain what's going on, but it gives you one last "treasure" to find.