Vienna is fantastic! I would strongly recommend purchasing the 'Vienna card', which entitles you to free access on the excellent transport system, which is essential as some of the attractions are on different sides of the city. Here's the link to more information on it:
I was there for three nights and that was a perfect length of time to accomodate what I wanted to see. I stayed in the a cool, small little hostel called the panda hostel. It was cheap, very clean and had a tram stop literally right outside the door, which took me to Westbahnhof to connect with the metro and bus routes.
Go to the Prater fun-fair if you can; the views from the famous ferris-wheel are class!
Although I liked Vienna, Salzburg was by far the best place I visited in Austria. You can do day tours of the ice-caves about an hour outside the city which I highly recommend.
I went from Vienna to Bratislava by catamaran along the Danube, and I'd highly recommend it as the optimum way of travelling between the two cities, if it suits your budget. IIRC, it was €22. It was dearer than the bus (€11, I think), but was a fantastic trip!
I stayed in Bratislava for 2 nights. It's a lovely city, with a nice blend between past and contemporary life/archetecture/culture. The one sore spot I had with the city was that on walking through the centre of the city, I saw this HUGE Tesco's supermarket. Way bigger then the ones at home, more of a department store actually. It just didn't fit into the culture and ambience of the city, I felt! I was inter-railing too, and Bratislava was quite cheap.
Hope that helps, PM
me if there's anything else you'd like to know!
[Edited 2007-03-28 12:37:08]