depends....
My experience is based upon 10 years of experience with various PUR's, 3 years of a commercial 40,000 per day (you read that right) unit that serves the marina I mmanage, and anecdotal experience with many of those in between. I personally have installed the PUR 35 and 40 on my boat and have maintained these and many others. First: It is GREAT to have a plentiful supply of excellent water, but yes, they are maintenance-heavy. There is NOTHING complicated about the maintenance, but you HAVE to do it or you will have a nightmare.....kind of like putting gas in your car's tank, changing its oil and filters.Don't do it and you will be sorry. I remember when Seven Seas Cruising Association did a survey which, after absolutely panning the PUR units, allowed as to how "those few who had managed to keep their's working had done the following"...all of which "discoveries" were thoroughly covered in the manual.....so READ IT AND FOLLOW IT!But, even a rebuild, yes, even a total. rebuild of a 40,000 GPD unit, is not a complicated task. My advice would be to read the manual, do the maintenance rigorously and without fear, and enjoy the water!Generally, the more frequently the unit is used the better.Run it in clean sea water. Daily to weekly, depending on salinity (not all sea water is equally salty) humidity, and temperature.Run it briefly on clean, non-chlorinated, fresh water occasionally if you are going to be in a marina for a little longer. Pickle it when you should. It is a very easy and quick process. The membrane will keep fine for months or even a year or so, depending on the manufacturer.Sometimes, you will need to do a partial or complete rebuild after a LONG period of inactivity as the seals, springs, etc., may have suffered. The handheld one you saw on TV is for emergencies, but PUR makes (or made) a unit that would put out a gallon per hour. A friend cruised with one and pumped for an hour daily and got his workout, as well!I bought PUR and have been happy with the units and delighted with their service. DC units like PUR and Spectra have the advantage that their energy does not necessarily require running the engine or using AC. Larger units pretty much have to run off of an engine. Although I have used PUR quite happily, I must say that if I actually needed higher output, I would think hard about Spectra. It has gained raves among real-life cruisers. It is more expensive however. A cruiser named Gary Albers has written a very good general manual on watermakers. I know you can get one from PUR, but it is not their book, so I imagine it might be available elsewhere. If you can't get it, I can get you his email address. I strongly suggest you read it. I am inclined to agree with the comments that you not buy until you are really thinking of needing it. Although the technology is not "high" (basically pumps and filters by other names), improvements are made. But, I would not leave it till last, as installation can require some planning and thinking. Plus, I happen to think that everything you use while cruising should be onboard, tested, and ready for some months before departure.By the way, I am based in Cabo San Lucas, Baja California, Mexico, and have done most of my cruising in California and in Mexico.And, in the Sea or Cortez, where water is hard to come by, my watermaker was near the top of the list of "musts"! Good luck, enjoy the water, and don't be afraid of the maintenace.