Difficulty
I initially duo'd with my husband, him at max skills in Gazluk keep gear, me as 55-58 rabbit / 65-66 psychology, ~ 476 health / 334 armor, in nearly all yellow level 50 gear courtesy of the last couple holiday events. As expected, we didn't really have any difficulty with the regular mobs unless we did some really stupid stuff. My dps against the mobs at the top seemed fine and I think I would have done fine soloing that area.
When we got to the ratkin, I felt they were doing a lot of damage to me - when I had two on me for a brief amount of time, I almost died. Since we were usually getting two or more ratkin each pull, I suspect I
really don't want to solo this place with my current build and level.
I later brought another character to solo: 62 cow / 67 druid, ~ 625 health, 742 armor, primarily level 60 purple gear. I did largely ok, and managed at one time to bring down 6 ratkin that just kept showing up one after the other, but also died multiple times to a 3-pull of ratkin.
The difficulty of the sewers feels too high to me because I have previously done all 3 of the solo Gazluk caves (minus Murklinger) on that same cow character, only obviously slightly lower level and possibly slightly worse gear since I think some of my current gear came from the caves. I hadn't brought the rabbit to Gazluk before, but I tried the Tower View cave with her yesterday and I did great. I died a couple times initially while I was trying to figure out how my skills worked solo, and then was able to carefully do the entire cave without a further death, including the Hound.
So, the presumably higher level Gazluk caves feel
much easier to me than the sewers. I guess it's possible the caves are too easy, but eh, I like them and I can still die there if I'm not very careful. I don't think my problem with the sewers is the health of the mobs there or even their damage - I think the problem is the spawn rate. In the Tower View Cave, I can carefully pull so that I will get at most 2 mobs at a time. In the sewers, I have mobs spawning on top of me and wandering into call range of the mob I'm currently fighting, so it feels I have no reasonable way to control pull size. I haven't learned the new precognition ritual yet but since I have mobs spawning right in the area that I cleared 5 seconds ago, I'm not really sure where I'd try to backup anyway if I see a new spawn coming.
I'm generally not fond of high spawn rates - I acknowledge it's great if there are lots of other people around but otherwise it just feels frustrating to me if I have mobs spawning right behind me, so if I reach a dead end, I have to reclear what I just killed a very short time ago. (I don't much care for the lower yeti caves for the same reason, although I love the Animal Nexus - maybe because that one is just one straight line with no backtracking unless I die.)
I'm not really sure if the difficulty or spawn rate really needs to be changed. It's doable solo, just takes more effort than I'd expected. It's nice for duoing and smaller groups, and I kind of enjoyed watching some level 70 people get smacked down for not paying attention and pulling 10 mobs at once. But ... I don't really see myself going there to solo at level 55. And especially not at level 60 because the Gazluk Caves are definitely an option then. Of course, I've done the caves dozens of time now and it's good to have another option.
Oh - the sewer bosses seem fine since they are intended for groups. My husband and I duo'd the spider together, although he came very close to dying. We had a group of 3 against the snake master and wiped miserably because we weren't really prepared for the adds. I think we would have managed to kill him with another attempt or two, but when we got back, there were more people killing him, so he went down easily.
Loot
I love the loot from the ratkin area. Lots of new and exciting stuff, and I love having barrels with random loot and really like the new cheese enhancing potions.
I also like being able to butcher the treants for wood, nice touch. ( I wonder, since ratkin are intelligent, shouldn't we be able to extract their skulls for phrenology analysis?)
I really don't like the mobs with no loot. It always feels to me that I wasted my time clicking on the corpse if there's nothing on it anyway. When we first went to the caves, it felt like we'd killed at least 20-30 mobs before I got any loot at all because most slimes don't drop loot and my husband got lucky on the round robin and looted the rats, while I got lootless slime after lootless slime. I was frankly sulking by the time I finally collected a single snake scale.
The slime and snake area at the top feels like it really needs some reason for people to want to go back there - ground spawns or unique loot or additional mobs that can drop something cool or ... something. When I went back with my cow, I mostly just grabbed the key and didn't bother exploring the rest of the area since I already knew there was nothing exciting there and the cartography xp wasn't worth the hassle to me. Difficulty-wise, the slime area seems great for 55-60 but since I primarily got no loot or some white junk, it just doesn't quite seem worthwhile.
Likewise, the water crystal area seems a bit boring because I haven't gotten anything I recognize as shiny important loot from it yet.
I'm not so excited by the gear drops which I think is primarily because of the Gazluk Caves again. I know where to to get lots of easy 60-65 gear, so getting hard 55-60 gear is just not as cool.
Atmosphere
I very much like the atmosphere of the place. The infinite slimes look awesome, and the water crystal area is really pretty. I like it that the different areas have different themes, and I'm always happy to run into more goblins to chat with (and I loved the "take the OTHER right" directions

). I also like the big dungeon design with lots of places to explore.
Miscellaneous
I don't like the druid-only stuff. When I initially encountered the swirly leaves and the book, I thought there was some exciting mystery out there that maybe I could solve with the aid of the goblin research or by learning a new spell or something like that. When I brought my other character and discovered that it just seems to be a druid-only thing, I felt pretty let down since my main character will never be able to read that book and learn that bit of lore. ( Of course, I haven't done the whole dungeon yet, and if there's a way later for non-druids to eavesdrop on a bit of the druid lore, that would be great.)
The standing water is a very neat touch, but I was surprised to discover that some of my abilities didn't work in water. Healing Sanctuary makes sense maybe, but Bun-Fu Blast? Really? I definitely feel for the fire mages here. There are generally enough spots to keep my feet dry but it takes some effort. (Which isn't bad, just ... different.)
The treant at the top only dropped a single key for our two-person group, so we had to go back later to get a second key. This might get tedious for larger groups? I'm also wondering - if someone is soloing the area and comes to the treant room after someone else already killed it and it hasn't respawned, how will they know to come back there for the key later? I'm possibly overthinking this.
I like the design of the dungeon and how it's very fast to reach different places from the entrance if you know where you are going and have the key. It's very nicely done. It's also nice that the sewers are right in Rahu, for easy selling and going back in.