I've played only Evil Original Campaign and I'm half-through Good Original so far, but I'm eager to make a topic to discuss and compare. I also tried a little bit the Price of Loyalty campaigns, but never finished even the first map.
I have a feeling the PoL campaigns were made in a way far too easy. I mean, in a campaing about wrecked ship I wasn't able to loose a single troop unit. My initial army was simply too strong! When I conquered the first town this way, I grew bored.
Both original campaigns have my greatest love. The choice-tree of maps sequence was a pleasant surprise, something I never met in Heroes III or IV.
I also noticed that Evil version is somehow more difficult. In my play as Archie's field marshall I had a hard time, especially as I blindly chose the less benefitial options (save Necromancers and look for the crown).
Movie sequences in between maps, as well as turning point option, are very pleasant. I much more prefere Archibald over Roland. He's far more likeable!
It's also interesting and well-set that the Dwarven Kingdom map in Succession Wars is the same for both sides! This kind of attention to little details and realism is what I value here, and sometimes lack in Heroes III or IV parts.