Several previous threads, but basically, for a cruising sailor, not very cost-effective, a) because they have a limited and relatively short (5 years or so) life and b) because the high-purity methanol is excruciatingly expensive. Most interest from racing types, where weight is at a premium.
I installed one about ten years ago. I wanted something that would sit in the background and keep the batteries topped up if wind/solar failed to deliver for any reason. The fuel cell has been good at that but you have to be realistic about expectations - I wouldn't see it as an alternative to a diesel generator. Capital cost is high but running costs per amp-hour have been quite modest. It has been totally reliable, needs no servicing and is pretty much silent in operation. The big downside, which I did not realise when I made the purchase, is that over time the platinum catalyst coagulates and efficiency degrades. When my cell was first installed it delivered 5.5A but this now down to about 3.5A . Okay, it still works but no longer as useful as it was. I would have thought it would be in Efoy's interests to offer a catalyst recycling facility to prolong the life of the product but sadly this does not seem to be part of the offer. Would I buy another one? Hmm maybe....