I use Niki regularly from Vienna to Moscow and back. My company would actually pay for me to fly business class with Austrian if I wanted, but I like the pre booked pepper chicken and spatzle for 8 Euro, which more than makes up for someone sitting in the middle seat and of course its a third of the price.
Even better, last month I had to delay a return by one day, the night before the original departure. Called the call centre, representative found the lowest cost comparitive fare to my original booking which resulted in a difference of 3 Euro plus a 50 Euro re-booking fee.
Absolutely magic ! you can't even do that with anything but full fare economy on the legacy carriers.
Niki/Air berlin are the absolute model for short haul airlines IMHO