I used to use them a bit in the past on this route, as TXL is usually more convenient than SXF where DY flies into. But the way SK handles traffic to and from TXL is borderline amateurism. I also encountered massive cuts in schedule, making day trips impossible and I also encountered the long waiting times before finally being able to check in at a desk (which must be done if traveling with luggage, unfortunately). As such, my business to Berlin went to DY on this route (with 3 daily departures to/from OSL directly).
Agreed that it's remarkable that they couldn't do anything with the void AB left and that apparently EZ offers such serious competition that they leave Berlin via the back door...