They are kind of a pain due to slower fueling ..but they do work. I have a original large aluminum baja filter and I use it quite a bit. If a fuel dock gives you greif ask to run the first 20 gal. Through it then check the results. If it's clean decide if you want to argue for its continued use. I even use it a home to run suspect gas through prior to dumping it in the riding mower. They really do a great job of getting junk and water out of crap fuel.
On our across the pacific trip I filtered all fuel and never had a fuel issue..and we got fuel from some very sketchy places.
I goy my current large one on ebay for about $75.00
We used our Baja Filter religiously for 3,000 hours on our single engine (FL 120) Defever 41 while out cruising. Never had a fuel problem. They are wonderful. West Marine used to sell them but no more. I don't know if they are available any longer. There are imitators out there. They may be ok, but the original is dynamite.
They work up to a point. But are in no way a substitute for good on board filtration. So unless you are traveling well off the beaten path or in an area known to have very contaminated fuel I don't see the point. I've gotten fuel from here (FL) to the West coast of Costa Rica and in between and never had the need to use one.
Here's for those who haven't seen a Baja Filter. Nothing will get through the 3 filters. The only down side is it's real slow but great for checking fuel quality or transferring from jerry jugs.