I will say I used a buyer's broker to find and purchase my current boat, which was fantastic. It cost me nothing (he and the seller's broker split the commission), and after a good conversation the guy knew exactly what I was looking for and was able to point me to several boats which were good candidates - and steer me away from several which looked like good candidates online (he'd already gone thru them and found significant issues). It REALLY made the most use of my limited time when spending a day looking at boats, and as he was a very experienced sailor and boat owner he was able to answer a lot of my questions authoritatively, put unsound impressions to rest, and offer strong alternatives such as similar boats. I found making cold calls to brokers to visit their client's boats time consuming, and their knowledge often lacking. Some were just terrible. My buyer's broker took them out of the equation as far as I was concerned - I never had to deal with them, or the owners. All offers went from my guy to them. He also had a good feel for how open other brokers would be to negotiations, which saved us time. I'm sure its dependent on the person, but I found the experience HIGHLY valuable, and would use a buyers broker again.
The other takeaway from looking at a bunch of boats in a relatively short period of time was how NOT to sell a boat. While buying my current boat, I was trying to sell a 1974 Catalina 27 myself. I figured no broker would be interested, so I placed ads in all the usual places with lots of pics. After seeing a few boats whose owners (and their brokers) made little or no effort to "stage" the boat (e.g. get all your crap off, and neaten things up a bit cosmetically), I went to a significant effort to do as much as I could with my old boat. Trust me, if someone brings their wife along and your boat isn't straightened up, its no sale. It still took a year and change (no one looks in fall and winter), but eventually someone put cash in my pocket and sailed away with the title to an old girl with some life left in her. My point is, shop a little if YOU are selling, and you'll get an idea of what others are doing wrong, so you don't make the same mistake.