Has anyone flown Air Berlin? I'm looking at possibly flying from VIE to DUS and my choices are Air Berlin for 99 euro or Austrian 199 euro. Everything else requires a connection. Thanks.

Air Berlin is one of many new Budget ailrines flying in Europe, same concept as Southwest. Basic service. Should be fine.

Have not flown them, but have read of them and seen the planes at he gate in both Dusseldorf and Berlin. They look quite nice and I would not be concerned in the least. They have been quite active in marketing this airline recently. I would fly them without hesitation. Not sure what they fly, but looks like a 737 or Airbus of that same size?

Air Berlin flies Boeing 737s . It has numerous routes now in Europe and seems to be getting a good reputation. Remember the price quoted probably does not include taxes and other charges that can be steep.
Austrian recently experienced personnel strikes on several days. While this has not happened again for several weeks, the situation is not resolved.
Austrian recently announced a policy of no food service on flights of less than 90 minutes( about the time vie to dus) and will charge for drinks other than water.
Air Berlin, no frills, does not sound too bad under those circumstances.

Thanks for the comments. I went ahead and booked with Air Berlin. The first price they quote does include the taxes so it was 99 euro total. Thanks again.

Air Berlin is better than most budget airlines. You get assigned seats and a meal and drinks on most flights. We flew them FMO - PMI - NUE - FMO and had perfect flights at low costs.