wait for winter and watch the s-bahn. Actually, it's not the BVG (City owned) that has major problems but the S-Bahn which belongs to the Deutsche Bahn. There was a major collapse in summer 2009(?) when the authorities realized the the S-Bahn had been saving money and cutting corners on maintenance for trains and basically compelled them to take 2/3 of their trains out of service for emergency maintenance. They still have not recovered fully, some lines are still serviced with under length trains or on a schedule that's half as often than actually planned. Whole lines getting shut down due to lack of personal or trains. Berlins public transport used to be outstanding, now it's sub-par.
I always wonder what basis for comparison people have when they complain about Berlin's public transport. Because, as somebody who's witnessed first-hand how it works in nearly every major metropolitan area in North America, and many across Europe and Australia, I can say without doubt that the situation in Berlin is phenomenal. A few slow or cancelled S-bahns in the dead of winter do not make a system sub-par.
It's not 'a few slow or cancelled S-Bahn'. Last winter season the system was so strained that the S-Bahn to the airport was running only every 20 minutes and was pretty much guaranteed to be either late or cancelled. So to get in time for your flight you had to leave like an hour earlier than planned. It's slowly getting better, but the last couple of winters you pretty much could not rely on the S-Bahn if you wanted to be anywhere on a specific time.
Compare it to Munich, where it's a minor scandal if one S-Bahn is like 5 minutes late. That's outstanding.
"Every 20 minutes" for an S-bahn is totally fine! Especially compared to cities like New York, where every 20 minutes for some U-bahn equivalents would be considered good.
Every 20 minutes on one of the main lines is not fine. Even if it were fine, it's only fine if you can actually rely on the schedule, but not if you need to take into account a high chance that it's '1 hour wait and then three trains in direct succession.' Slow, but reliable service is ok. Slow and unreliable is not.