Yes, above waterline it is what I use about 85-90% of the time. Stays flexible for well over 30+ years and does not damage your boat trying to get stuff apart. Many quality builders used to use butyl tape but it was labor intensive so they stopped. Our CS-36 was bedded with butyl at the factory in 1979 and to this day over 80% of the deck fittings are un-rebedded and not leaking. I R&R'd the chain plates this past spring, just to check on them and have them inspected / magnafluxed, and even at year 31 all six of them were still BONE DRY, 31 years!
Find me 10 boats with chain plates bedded with polyurethane and I will show you at least 9 boats at 30 years leaking. I know it may sound funny or odd but on the CS list we don't even discuss re-bedding as it is not a constant problem. Really I can;t recall teh last time re-bedding even came up or deck leaks. When I compare that to the C-30, 34 & 36 lists, I have owned a C-30, 310 & 36, there is a huge difference in the number of discussion revolving around wet decks and re-bedding. All of my Catalina's had leaking hardware and I did a full re-bed at year one on a BRAND NEW C-310 due to multiple deck leaks. This is unacceptable.
Butyl is far more flexible and flexibility is what you want in deck hardware. I had so many requests for the stuff I actually found the original manufacturer and the original compound used on my boat and ordered about 5k of it to sell to other boaters who are interested.
All sealants will work if installed correctly but butyl remains significantly more flexible so of there is some movement it does not fail as easily. 90% of the leaks I see are installation related not so much product. I prefer butyl for all of its positive attributes.
My favorite "sealant" was 3M 101 but they discontinued it..
If you are interested in getting some let me know: http://sbo.sailboatowners.com/index.php?option=com_marketplace&page=show_ad&catid=19&adid=2385&Itemid=204
Sorry, I never did finish that article on my site, DOH'..
Here it is with a better write up in my forum on SBO: http://forums.sbo.sailboatowners.com/showthread.php?t=117172