I've done pretty much all of the tastings and always had a good time. It does indeed depend greatly on your cast member who is doing the tastings, but it's usually a member of the bar staff who knows their stuff pretty well. They like to put a menu / check folio in front of the bottle for a "reveal" after people have guessed what certain liquors might be, so that's fun.
Note that a lot of the hard liquor tastings are at 10:00pm on a lot of the ships, so you're going to be tight if you have second dining. And yes, you do leave "well lubricated" from those... :-D
My absolute favorite has been the rum tastings - I'm a huge rum fan. But I've learned a lot at the whiskey, scotch, bourbon, etc tastings as well. Wasn't a huge fan of the mojito one, but my wife likes those, so we still had fun.
Mixology is a lot of fun if you can get into it - it's very popular. On some of the longer cruises, they do chocolate and liquor tastings - HIGHLY recommend that. Insane what they can pair up there. And some of the best chocolate you'll ever have too.
Cheers!