We were upgraded to the Explorer Suite as we were the only couple on board except for the FSKH GM plus family. We had a look at the cabins and they seemed quite small. Now, we barely spent any time in our room, so we probably wouldn't have minded, but on a full boat it might get a bit cramped and the extra space of the suite might come in handy.

We did the 3-night cruise and wished we had done the 4-night one. I don't think we would have enjoyed the 7 nights though as the permanent snorkelling (or diving for my +1) does get exhausting.
I did get a bit sea-sick while crossing from some atoll to the next but it wasn't too bad.

German tour operators still offer a 2for1 (which we took advantage of) in the off-season when also booking the hotel(s). I think that includes the suite.