Karl's Place is great. It's a very comfortable place to stay, with Lidl and Aldi (well, Hofer in Austria) very close. It's easy to get to from the airport by train (on the S7, getting off at St. Marx). The flat itself is very easy to check-in to and has everything you could wish for, including a dishwasher and washing machine.
We walked into Vienna from the flat, which took about 30 minutes, but the metro is also nearby.
We'll be going back to Vienna at some time because it's a lovely city, and we'll definitely be staying at Karl's.