At the Island level, I do think Sentence Island is useful for discussion of the terms. My son also really liked the story.
I don't think Building Language added all that much. You can easily wait for Caesar's English (which has been really great here).
The poetry is well done, but we kept on not getting to it.
From Level 2, we didn't use poetry or much of Paragraph Town (although again, my son enjoys the story).
And in Level 3, we haven't touched poetry or Essay Voyage.
I do really like CE 1 and 2, so I would have missed out if we didn't use it.
For level 1, I think we would have missed not having Sentence Island (again, story & reinforcement).
We've mainly used WWE and now WWS for our writing, but CE for vocab & MCT Practice (Grammar and practice books) for grammar.