There is a lot of variety to pick from - all kinds of spices and vegetables which have been layered between slabs of essentially chilled scrambled egg. On this occasion I chose picantes (peppers), mushrooms and paprika.
The bread isn't anything special worth particular mention - it is untoasted and quite plain. A nice sourdough would be a better option. The topping is as expected if you read the above description - chills scrambled egg with a particular flavour injected. It really isn't any mindblowing experience that others seem to gain from eating them.
Next time I would order whatever topping you want. It is certainly fine for a daytime snack while wandering through the lovely central streets of Vienna, but I wouldn't go out of my way to find it.