Yes I have 555's on my boat too. Before I purchased, my research found lots of mixed feedback (ie good and bad). The point on them being old and getting hard to find parts was universal though!
What seemed to be a trend is that the bad feedback was from installations where they were being driven harder - higher rpm, turbo, express cruisers, etc. The lower stress installations had the great feedback - 20,000 hours, run forever, etc.
My conclusion (and hope!) is that my lower stress, non-turbo install is the latter. I have twins and run them at about 1900 rpm, so far below what express cruisers would. So far so good!