I actually finished them all, most on the day on which they were published (I had the entire day, since the problems open at 06:00 in my timezone; I didn't try for any leaderboards :p). Some problems took me far too much time (day 15 with its huge simulation!), and overall I agree with the general sentiment that the problems were too finicky, with a lot of details to implement without much real thought.
Day 23 part 2 (teleportation nanobots one) was fairly difficult as well; I tried to implement an actual solution, but failed because of wrong assumptions about its correctness. (I tried to represent the intersection of a number of octahedrons with the bounded volume of 8 planes, but it seems that representation is redundant, so there are invalid configurations. I didn't feel like fixing that anymore after 8+ hours.) Looking at the solutions board on reddit, I wrote a smtlib2 script for Z3 that solves it; it works, but isn't all that satisfying.
The sheer amount of pathfinding amazed me; some of the plain pathfinding problems were good (day 22 with its pathfinding in the implicit graph where you split nodes up into the various combinations of tools), and I was happy to see that day 25 was just an unordered graph component analysis (i.e. a few floodfills).
I liked yesteryear more, and in general I'd like to see smaller problem statements (and thereby also less details to implement!), and possibly slightly more of an algorithmic challenge, though the latter is definitely not necessary (this is not a competitive programming competition, it's an advent calendar of all things!).
Still, big thanks to Eric for giving us this!
(I did the problems in Rust, to become more familiar with the language. See my implementations here[1].)
[1]: https://git.tomsmeding.com/AOC/tree/2018/src