Jaws of Hakkon: This one adds a new area to explore (that looks a bit like a cross between the Arbor Wilds and Crestwood) that contains a ton of tier 3 materials (some new some old), a few new tier 4 materials, a new ability for the Inquisitor that deflects projectiles, various new schematics slightly more powerful that what’s found in the default game, Astariums to unlock, skull shards to collect, a few War Table missions and agents, more Fade Rifts to close, and new Skyhold decorations to buy. It’s got a lot of stuff and for the most part fits into the rest of the game seamlessly, with the only real oddities being the tons of loot you find when closing Rifts and there being silverite scattered around instead of stormheart. Word of advice: Don’t take Sera with you when doing the quests here, as many are spirit-related and she does not like spirits.
The Descent: This adds a new underground area to explore that’s not really anything like anywhere else which just so happens to have two immediate annoyances. The first it that using some (any?) texture mods causes the cutscenes that happen here to freeze the game, while the second is that to actually start exploring the Deep Roads Cullen cannot be assigned to any current War Table mission. That’s because there’s a special War Table in this area you have to use to unlock areas and do missions down here, and only Cullen can be assigned to the first one (you’ll need 64 power to eventually open all the side areas). Other than the new War Table, the area has some new enemy types, new schematics more powerful than the ones in the previous DLC, and a series of mugs to collect (like the main game’s bottle collecting). It does not have any new materials though, with just the same resources that you’d find in the Storm Coast scattered around spiked with a bit of silverite.
Trespasser: The storyline portions of Trespasser remind me quite a bit of the Witch Hunt DLC for the first Dragon Age game (i.e. a sequence of somewhat forced events that ends rather abruptly). Thankfully, there’s more to this DLC than just the epilogue it tacks onto the main game (the new ending slides aren’t even narrated). The actually useful additions are the new ability upgrades you can switch between, the new Sigil loot that can be attached to most armor, and the extremely welcome upgrades to various quest and Wartable rewards; all of which have been inserted into the main game. Also possibly of note is that the ending makes it appear the next game will take place in Tevinter.
So, are any of these worth the $15 price tag? No, not really. Jaws of Hakkon comes the closest probably due to sheer amount of content (repetitive though it may be), but you’re best off just buying the Game of the Year edition which comes with all three (along with some weapon/armor packs) for $40.