Thinking about getting it on the fence.

If you liked the original game, then buy it; it's fun exploration and more story.

If you didn't like the original game, then don't. :-)

Basically what he said. White March also adds 3 new characters and lots of other nice features. Soulbound weapons are different and the forge adds a new way to enchant equipment. It's not just more of the same, it also reflects a little bit what they learned since making the original game.
I'm almost at the end of Part II. I enjoyed playing it and I liked it that they once again gave most quests multiple solutions. I'm interested in learning how it'll turn out and how it will influence the ending slides.

Does the White March add a new story at the end of the basic game (like an expansion, e.g. Throne of Bhaal for Baldur's Gate 2) or "just" more content for the basic game before the ending (like Mass Effect 2/3 DLCs) ?

It adds more content for the basic game before the ending (like an expansion, e.g. Tales of the Sword Coast for Baldur's Gate :-P)

Entire White March (pt. 1 and 2) adds between 25-30 hours of content set in an entirely new region + 2 new locations to the map of the base game. It rises the level cap to lvl 16 and allows you to choose whether you want the third act of the base game to be more difficult to compensate. It also adds a new type of items called "Soulbound" which can be bound to characters with certain classes and then level up and gain new abilitities - so that's to the extent of that, whether you call it expansion pack or DLC is up to you.

Anyway, if you're worried about having to replay the game, you don't - as long as you held onto your old saves. The original Pillars of Eternity has automatically created a hard save before you enter the point of no return in the epilogue, and if you wish, you can load that save and use your party from it. Upon entering White March, you'll be asked whether you'd like the expansion to be switched to a mode for lvl12 characters.

I only started playing basic PoE not long ago, so not really worrying about anything, but yeah, I wanted to know if it's Throne of Bhaal or Tales of the Sword Coast style expansion. Thanks. :)