I had a game where I had almost all the foreign cities covered by a privateer (some had to tackle multiple cities, the most I could get was 5 with one) and it was pretty cool. I got tons of experience from it and my gold per city was typically 2 per turn, with a few 3s and occasional hauls of 20-30 for a newly blockaded city. My end stats said I had built over 60 of the buggers and lost about 20 of them (DAMN good) and I popped 3 GGs before even declaring a war.
It was apparently so effective that I had no real worries from anyone else to let me win the game, except India, who had developed Frigates early enough that I couldn't risk blockading them at all, and I am CERTAIN that this forced (or at least sped up) all three of the "please vassalize us" offers I got before the Indians started flooding the oceans with Figs and SOLs on behalf of everyone, hunting down my defenseless Privateers one by one. (Combat 3-4 Privateers sitting on the coast are defenseless? Yes they are; I don't know why, but I hardly ever win with them against undamaged Frigates.)
Despite all this joy I had, I still can't say that Privateers are worth much. They are outclassed far too easily and quickly, and if the AI would actually USE those hoards of Caravels they build to counter them, then even then it would be too expensive to maintain them. About the only use they have is for kicking someone who's already down, and they have to be out there for a while to pay for themselves. Don't forget the extra maint and military maint they cost; this can easily average 1.5 gold apiece, meaning it's critical to find city clusters to blockade, which conversely just ups the odds of getting attacked.
If you could instead just designate existing units as being privateers, that would make them last a lot longer, and be worthwhile.
