From what I understand, the new core rulebook's content is mainly the same.
Does that mean the reference cards from the three boxed sets are still a viable resource? Can I let players use the boxed sets' booklets without worrying, or do they need significant errata to make them current?
I am thinking of purchasing the book so all the rules are in one handy location, and just want to make sure how much of older sets are still useful.
You could absolutely allow your players to use the existing Player Books from Sets 1 through 3 for reference without any concerns they'll be different to the Core. And, yes, all the reference cards remain a useful resource for play.
Basically, the Core Rules version represents a compendium of the old boxed sets reorganised for easier access and reference. I say this based on owning all the sets and the book.
You will find a couple of new specialisations added - specifically the Reaver and Necromancer, a few new creatures and spells, and a brand new adventure. However, essentially, if you have the original Boxed Sets you have all the rules herein - just spread out.
