Arrived to the Keflavik Airport around 12:30 at night, took about an hour to hail a taxi and got to Guesthouse around 2 am. Check-in is at the front desk of the hotel across the street, which was very simple.
You go up some steps, open a door, and you are greeted with a hallway with around 10 rooms, 2 bathrooms on the left side that are shared between all the rooms.
The room itself is practically a shoebox. It was clean, looked nice, and the bathroom worked as it should.
However, the only reason I booked this hotel was because I saw that it had a sauna, and since I only had one night in Iceland and wanted to recover after a late journey, I was really looking forward to it.
Finding out that it's in service and has been for a while at the front desk was pretty irritating. Not typically a karen, but when that's pretty much the main reason I selected a place and there was no note anywhere that the sauna is out of service currently, it's very aggravating.
Other than that, breakfast at the hotel across the street (came with the room) was incredible - so much variety, lots of healthy options, coffee machine with multiple delicious tasting grounds.
If you can afford it, Guesthouse is a good option. But be aware - the room is TINY. Even a single hostel bedroom is larger, and those go for half the price.