I picked up Downcrawl and Skycrawl, both by Aaron A. Reed. My overall evaluation of both is quite positive, tho' I expected not to like them when I first ran across their descriptions. A key source that gave me enough information to decide to buy them was this extract of the core Downcrawl mechanics that Reed makes freely available. I suggest you go read it to make up your own mind.
Downcrawl is 59 pages long, and contains rules for generating and administering an Underdark campaign built on point-crawl principles. The PCs move in abstracted journeys between "volumes" (collections of related sites) with tools for both generating complications and encounters on journeys, and for generating volumes, sites of interest within them, and their inhabitants.
Skycrawl is 75 pages long and uses the same basic system with a few tweaks and adaptations to generate and administer a campaign set in an endless expanse of aetheric-sky pocked with small floating islands. The islands serve the same role as "volumes" do in Downcrawl, tho' there is an additional mechanic to represent the islands moving around over time.
The journey system in each involves accumulating a resource known as "tack" through various activities (both abstract downtime activities and adventures) which combines with accumulated rumours (that the referee creates and hands out to players). The abstraction is such that most journeys, unless something goes very smoothly or very wrong, will produce 3-5 encounters moving between volumes or islands. The systems sit at a nice mid-point where they're not just "plan out three encounters and have them happen along the way" - PC choice matters - but they're also not so granular that you need to draw out the exact route that PCs use to get from one spot to another.
Both systems work using principles and formats taken from Powered by the Apocalypse. I'm not a great fan of the what I think is the modern format of PbtA "moves" where they are presented as self-contained boxes that begin with the conditions of their invocation, and the order to enact each procedure is either nested in another box or must be determined through careful reading.
With both PbtA games more generally, and specifically with Downcrawl and Skycrawl, I would prefer the addition of a visual element to the boxes that distinguishes top-level procedures (one that are not typically called as a consequence of another procedure) from procedures that are nested within others.
Downcrawl's moves are a little easier to parse of the two because most of the procedures for travelling are listed on separate pages from one another, or at most, a procedure and its most commonly called sub-procedure are on the same page (pgs. 10-15). Skycrawl's moves (pgs. 16-20) are a little more jumbled with several small moves hidden at the bottom of the page and referring to things that require one to flip pages to sort out. In both cases, the complexity is kept in hand well enough that some careful study will bring the relations into clear view, but for me it took reading Skycrawl's moves about three times before I started to understand firmly what move gets invoked when.
It's only a small usability detail, but it's also my most serious gripe with the book, which I think speaks more generally to how useful and well-done both books are.
The encounter procedures in both books are capable of producing highly varied results, using small nested tables built off a single roll of 3d6, with each die determining an aspect of the encounter. The tables proceed from general to more specific, more abstract to more concrete, and the examples under each result (typically four per) are a good mixture of inspiring and straightforward.
The tables are set up so that they are meant to be used during play, rather than generating random encounters ahead of time, so a referee will need to be comfortable with improvisation to make the most of them, and you'll want to note any unusual results beforehand and ensure you have suitable monsters prepared.
Each book contains a different alchemy system. In Downcrawl, you're mixing up harvested fungus to produce various drugs and potions, while Skycrawl has you gathering magical sediment that also serve as the main form of transportable wealth. Both systems seem set up to basically have one or two players who are really into them, while they can mostly be ignored by everyone else (Skycrawl says this explicitly). It's worth reviewing both systems before play and deciding what kind of magnitudes and powers you want to give these potions and drugs, since they're suggestive, much like the monsters.
I haven't had a chance to playtest either yet, but I will say that these books passed a very important pre-playtest threshold, which is that they made me want to use them in a game. I'm tempted to adapt them to Openquest (the new 3rd edition just released to backers - a review forthcoming once the first round of backer revisions and errata is incorporated into the text) and run a short Downcrawl campaign as soon as I can free up the time to do so.
Once again, I'd recommend checking out the extract above before making up your mind to purchase these. If you do like one, you'll probably like the other, so I'd recommend getting both at once.