The life is certainly not for everyone. I haven't done any kind of scientific tally, but it seems like about a third of the boats we've met just starting out to do long term cruising give it up for one reason or another sometime during the first several months. But for us, it is heaven. It surely has its moments of excitement (waking up in the middle of the Pacific with smoke filling the main cabin comes readily to mind!), but by and large, it is a much slower and more relaxed lifestyle.
To say that it is stress free would be a lie. At one point during our first (month long) ocean passage, I had made the decision that as soon as we made landfall, either my wife or I was permanently getting off the boat. I would let her decide which one, but one of us was getting off. Fortunately, by the time we actually got there, we had worked things through and our relationship is stronger than ever. We've done several passages since then, without similar problems.
Who succeeds? Boats where both are eager, contributing partners. Boats where one is in charge of doing everything and the other is pretty much a passenger seem likely to fail. Also boats where this is the dream of one of them while the other has simply agreed to come along an give it a try seen less likely to continue cruising for long.
Also, those who are prepared to fix anything on their boat or do without it are far more likely to stay with it than those used to picking up the phone and calling a repairman.
I am really not trying to discourage anyone, just hoping to encourage people to go into it with their eyes open and with perhaps a plan B as for those whom the cruising lifestyle is a good fit, it can be heaven on Earth.