It was a great series during the start, and it got even better when they started finding out about the island itself.
The ending felt rushed, similar to what happened to Psyren when it was cancelled. I'd have preferred something less open-ended, but I guess it was alright for what it is. Some readers have suggested reading Yamada's earlier manga series EX - Shounen Hyouryuu, as it's seen as Cage of Eden's spiritual successor in a way.