**Warning: this post may contain spoilers as it talks about stuff that happens after the main game ends. Also it may digress into discussions paying for content.**
So far, the only downloadable content for PoP worth speaking of is the Epilogue. It picks up exactly where the ending left off and carries the prince and Elika away from Ahriman's immediate control and into new lands. It also serves as a way for the narrative to explore the now damaged relationships and changed positions of the characters. The prince has found a faith (of sorts) while Elika has lost hers. She's also a little ticked at him.
The DLC is a couple of hours long, introduces a new power, a new acrobatic combination, and a new combat move.
The power is a variation on the Red plate from the main game (purple now) that launches you and causes temporary wall sections to appear. This is oddly pointless because there is no other way through, and they always appear long enough for you to use them.
The new acrobatic combination tacks the wall grind onto the end of a wall run. Not that interesting, just never used in the main game.
The new combat move is a QTE that you can initiate to rush the enemy. I could only get it to work sometimes and didn't find it that useful, because I spent most of my time trying to lure the enemy to a cliff edge, not rush towards him in the middle.
Narrative-wise, the DLC felt fairly satisfying. It answered some questions, posed some new ones, and did a much better job of setting up a sequel. The layout, however, left something to be desired. The epilogue is entirely linear (except one small 3-way puzzle section). There is little exploration, and all of the false choices from the game that provided a feeling of openness are gone. The difficulty has also be increased a notch, with some acrobatic sequences having an excruciatingly long time between checkpoints.
I'm also a little conflicted about paying ~$12CAD for the last chapter. I'm not sure that's a good precedent to be setting. I don't want to have to pay for each installment of a full-price game. Sure, the game didn't actually need it, but I have a feeling that the sequel will probably assume that it's cannon. I'd probably recommend you buy it, if only for the story. Or at least play it at friend's place if they've bought it.