A lot of people will say they barely work at all, but DS and I have used them for 4 cruises now and had great success with them. The only times they got iffy is when DS and I were really far apart, say one on deck 10 and one on deck 3, and both deep inside. But those times were few.
I guess the best thing I can tell you is to get some form of Motorola Talkabouts. I got a great deal on some Cobra's, and they were utter garbage. Back to the Motorola's!