The capsules are nice and offer some privacy. The room was simple but very clean and lockers were available (unfortunately my bag didn't fit in them). I could not find the key to the small locker inside the capsule.
The mattress was very thin and soft, so that you feel the ground. (I am 60 kg and had a decent night, but can imagine this might be a problem if you are heavier.)
The hostel entrance is fully automated, so there is no staff at the door (although they are easy to reach and very friendly by phone). However, this means you cannot get in earlier than the check-in time (15.00), to drop your luggage for example.
Overall I had a very nice stay at Vienna in a very clean and functional hostel that offers some extra privacy in the capsules.