Yes, just once. I had a 2 day conference there many years ago. I tried to bunk off and do some sight-seeing but I ended up sat near my CEO!!
One day I shall return, at Christmas time. And perhaps get tickets for the New Years Concert in the Phil. You can stand at the back for 30 Euro or get a decent seat for about 1,000 Euro. The cheap seats sell out years in advance, as do the very expensive ones. The mid range go rapidly too.

oh thats bad, only 2 days and than a conference. when you will return one day you must take a little more time, there is so much to see 🙂
…and when you should come, give me a call and i show you a little from vienna 😉