This restaurant has an extensive self-service buffet where you pay by weight. There is a wonderful range of vegan and gluten-free options and a full bar. The servers are friendly and multi-lingual. It's in a great location on the Ring right across from the University of Vienna. Because it caters to students as well as the people who work in the government buildings nearby, the decor and atmosphere is upscale and edgy, but very comfortable, and we were definitely able to "come as we were" in terms of clothing. My husband and I both thought that the price for the food was extremely reasonable given the excellent quality. In short, this place is an absolute winner. (To give you an idea how much we liked the food: they have a cookbook that they sell to raise money for charity, and I bought it on the spot.)
A tip for rookies: The payment system is smart, but it takes a bit of getting used to. When you arrive, you're given a card, and then you go to the buffet, choose what you want, and bring your plate over to a scale to be weighed. The confusing bit is that you think you should somehow scan your card, but really you just need to press it flat against the upper left hand of the monitor. Then, when your plate has been weighed, be sure to press the green "Buchen" button on the screen. When you do, a receipt will print out and that's how you know your order went through. Bring the receipt and the card with you to your table, where a server will come and take your drinks order. It's a bit strange and it took me a couple of tries the first time, but it's very easy once you figure out how it works.