I went recently for a works do and it depends what you are looking for.
We went on the Friday eve to the 'bermuda triangle' which is a pedistrianised area of bars bars bars. It gets going late (we arrived about 11.30) and stays open until late
The next day we walked a lot (too much) but it was pretty. Doi hire a bike - cost 1E for an hour - from the street places (it is an automated system encouraging everyone to bike everywhere) The cycle paths are Fab, you can cycle around the 'ring road' in about 3/4 hr or stop along the way, leave the bike at another bike stop and walk.
We also went ot the 'sky bar' for a treat. It has fab cocktails, v v strong, great views and is a nice posh place. see sky bar Actually not great reviews but I thought it was a nice place for pre dinner drinks.
You MUST go to the PRATER \link{http://www.prater.wien.infohere} It is a great place for rollercoasters, rides etc. Some quite pricey, but again good fun. DON'T go on the swing type ride (Sky swing I think) I had Chronic Vertigo and was sh*t scared on it!! It is also traditional where there is a giant ferris wheel thing.
I liked the city it was quite compact, but a real mix of nightlife, fun and culture. I didn't find it too £££ except at the Prater