Definitely in my all-time top 3 game list.

Gameplay: you control one hero, recruiting units from various spots (e.g. towns, dwellings), following the narrative. Movement on the world map is in real-time. Battles take place in a turn-based tactical system. Game interactions also include casting combat spells, world-map spells, gathering and interacting with artifacts(!), and using some special abilities I won't mention here for spoilers' sake.

Pros:
- very high-quality and polished visual art style. There's no way I can highlight this enough. Just have a look at any gameplay video and you'll see what I mean.
- very good replayability value - all generated monsters and artifact presence/locations get randomized whenever you start a new game (so in one play-through you might come across an artifact that completely changes your play-style);
- fun mechanics - for example, some artifacts which have good alignment get 'upset' if you fight too many good-aligned creatures. The artifact must then be suppressed (which is done by battling its guardians) for it to be still usable.
- challenging tactical battles - for example, the battlefield uses _hexes_ instead of squares (which, as Civilization fans found, make things a lot more interesting, tactically).
- well-executed soundtrack.

Cons:
- the story can get slightly childish occasionally, but it's all in good fun.

Additional notes:
- I'm not sure why this pack contains both Armored Princess and Crossworlds. Crossworlds is basically AP + a few more campaigns
- play KB The Legend first, then play the 'Orcs on the March' campaign from Crossworlds. 'Orcs on the March' is basically 'Armored Princess' + an additional set of quests + an additional race (the orcs) + additional skills. Don't play Armored Princess standalone, you'll be missing out.
- later games in the series (Warriors of the North, Dark Side) are, in my opinion, not worth it - they tend to have a lot of bugs and their campaign isn't that memorable. Just stick with the games in _this_ pack.

Conclusion: I don't usually qualify something as a 'must-buy', but if one game (... pack) were to deserve this label, it would have to be this one.