I actually stayed at four places in Mombasa at the Sand Island Beach Cottages, which I have reviewed before.
The Mombasa Serena is a beautiful hotel, and the staff are attentive--I stayed there the night before my wedding. However, they said they would mend my trousers, and they didn't but still charged for it! And if you walk along the beach, you get accosted by the beach boys more often than on the Bamburi Beach. Also, as much as the monkeys are pretty to look at, they are a nuisance, and there is only a half-hearted effort by the hotel to get rid of them.