"I have played with them (
"I have played with them (mostly to see if the claims they make are true since alot of questions do come up) and as noted above I found the following:
They do get the "nose" down alot quicker so the boat comes on plane faster.
They really like to have the boat run in a straight line - especially noticable with a tiller model, so if you are making alot of (turns) it's like trying to steer your car in a parking lot with either standard steering or a blown power steering pump.
When you do go into a turn however, the boat really "hangs on" so it may be desireable for ski/board boats - you can do some pretty tight turns at higher speed without the boat (sliding) out on you.
As Jeff notes, they do put significant stress on the cav plate. I have never had a first hand problem (yet), but a few mfgs (Suzuki comes to mind who also built the small 4 stroke Johnsons) have had more than one busted cav plate due to aftermarket "fins".
I have never noted any "improvement" in fuel economy as a result of their use (one of the big selling points if you believe the writing on the package).
Mercury has basically no opinion on whether they are valuable or not and does not advise on their use either way - however, if it was a new motor and you ended up with a lower unit problem (during the warranty period), I'm sure they would be able to link the issue to your "aftermaket retro-fit".
After all my horsing around I have found they have a very specific, limited use for the average boater.
I like them if you have a smaller boat, with a lower horsepower motor (under 25 horse) and travel significant distances in no wake/controlled speed zones such as canals - by keeping the stern up/bow down at "lower speed" they really help with visibility - but can't really think of another "feasable" application.
If you "need" a fin then what you really need is trim tabs that go on the boat itself - these fins are just a poor man's (poor) alternative.
Next up the ladder would be products such as "smart tabs", which cost about 100 bucks and are a far superior alternative to the cav plate mounted fins (my opinion - every boat longer than about 16 feet should have trim tabs)....
Last thought - Merc, BRP/OMC, Yami, Honda etc spend millions in research and design. If cavitation plate mounted fins were a good/valuable product and drastically improved things, don't you think they would have incorporated them into the design of at least one model by now????"