Which DLCs are worth a look?

Here's what I think:

Jaws of Hakkon:

Mostly more of the same as the main game but fascinating in new lore. Expect unexpected relevations. Also the Frostback Basin map is more dense in content than most of the main game maps. So in spite of featuring the same playing style, I like it more than some main game maps. Downside: a mandatory difficult dragon fight.

Descent:

Impressive underground vistas, nice for explorers, and quite a bit of new lore about dwarves. Also, the best two dwarven NPCs of the DA world so far. Downside: it's ultra-hard in difficulty. Expect prolonged fights against sizeable groups of hit point monsters. I was reminded of ME1 on Insanity.

Trespasser:

This is your Inquisitor's epilogue, featuring exploration of unusual surroundings, fighting a qunari plot, and lots of new stuff about elven history. Expect your view of this fictional world to adjust a little sideways. There's also a setup for the next game. Downsides: you are very much railroaded through the plot with areas becoming inaccessible once you're finished with them, and.....the Inquisitor's personal ending sucks in at least three different ways. You have a choice in exactly how it sucks in one of these.

Trespasser is actually the one I wish I could nullify by leaving it unplayed, but unfortunately it's the part that most affects the Inquisitor, and does so canonically. If you want to see something different, landscape-wise, I'd recommend Descent, provided you don't mind extensive fighting. Meanwhile, Jaws of Hakkon features the best of DAI's typical playing style.