2 things: (i unfortunately bought a boat with a panther)
are they any good? well they're pretty low maintenance and as long as it works they'll be pretty reliable and with decent power you'll have a 50-60mph boat. i have done pretty much nothing to my pump, it runs great, no issues.
are they GOOD? no. i have one, theres ZERO parts for them. my only option is to do an energizer kit, which is about $2500 for the kit and im going to see a couple hundred rpm drop and maybe a 5-10 mph pickup... MAYBE. if you want to convert to a berk you have to do serious hull glassing since the intake on a panther is much wider than the berk.
my opinion, if you're looking for a boat... run away if it has a panther. if you already own one... how much does it mean to you to keep it? is it a family heirloom, or are you JUST starting out on a project? if the later is true, try and ditch it and find a boat set up for a berk. theres tons of great deals on berk jet boats out there from crazy rigged racers to weekend family cruisers, to full on projects with just the pump and hull.