We just returned from the Iberian Explorer cruise. We will start with the positive comments. Nice large cabins, we had a verandah suite, our dining experience was fine, nice choice of entertainment, easy embarkation and disembarkation, most of the personnel was very helpful and friendly.
The ms Rotterdam is an older ship which has been refurbished. We presume the refurbishing did not include plumbing and this was definitely an issue for us. The toilet in our cabin never worked properly in spite of the plumbers' efforts to replace every part possible. We also found that the temperature was very hard to control, most of the time, our cabin was very cold but the common areas were too hot. Also, if you are used to seeing your invoice information, etc. on the TV screen in your cabin, or watch the latest movies, this won't happen. They have four movie channels but they show the same movie all day. They still have VCRs and you can borrow older movies.
We would not discourage anyone to cruise on the ms Rotterdam as you might not encounter the same problems as we did and after we were finally given another cabin, we truly enjoyed the rest of the cruise.