I was almost done with Korsakovia and about to start Dear Esther last year when the hard drive they were on died, and I keep not getting around to reinstalling them. Maybe this will finally get me to do it.
The introduction of Korsakovia is amazing and probably the best thing (to me, at least) I've seen in any horror game ever. The combat (when there is some) kind of sucks, but it turns out to be relatively easy once you figure out how to hit things. It was pretty frustrating before I got the hang of that, though, but nothing compared to the goddamn jumping puzzles, which nearly got me to stop playing. Those are the main problem with it. They're not fun, and they're not scary, and they're only hard because trying to pretend you're Mario when you're actually Gordon Freeman (not in a story sense; in a same engine, same point of view, same controls sense) just doesn't work, and this is coming from someone who actually thought Xen in HL1 was fun.
If you can get past that, though, the atmosphere is amazing, and the bizarre level design will screw with your head. It's also neat that it's (somewhat) based on a real neurological disorder [http://en.wikipedia.org/wiki/Korsakoff%27s_syndrome], and as someone familiar with it and how things like that work, it was pretty cool seeing it handled surprisingly well (if bent a bit for storytelling's sake) in the game, better than most similar things in TV/movies. It's worth checking out just for that and the way it's presented and develops over time. If you have to noclip your way through a couple jumping puzzles (as a last resort, preferably, since sometimes you can't do it just because that's not where you're supposed to go), so be it.