Does AB operate its own lounge at TXL or is the BA lounge the only option available?

Do they serve a decent breakfast with any hot dishes? I will need to leave my hotel at the crack of dawn to get to TXL more than 90 minutes in advance of the flight and that leaves no time for breakfast at the hotel.

Only BA. Not sure about food options.

I have never used the BA lounge in A terminal but Air Berlin also has an "exclusive wait area" for status members in C terminal. It's over by Gate C38, you can use the Fastlane on that side of the airport and use the bridge inside the security area to get to any of the gates, though it's a bit of a walk up and over the baggage area. To get in, you need only your card or the ticket which will open the doors to the lounge. It's also upstairs so if you have heavy stuff and don't want to bring it up and down then don't bother going there. There is no elevator.

The "waiting area" has only a coffee machine and drink machine. There is also the normal newspapers and stuff and sort of cafe style tables. There are power plugs around the rooms as well.

I flew last week TXL-AUH and was told to use the AF lounge. At that time (around 21:00), BA lounge was closed and AF was open but closed at 21:30, so I guess they keep it open for the AUH fight.

You can use the BA Lounge or one ground above the Air France Lounge.
Both are ok for Chilling and enjoying a hot beverage, but the breakfast Options are very limited. No hot dishes and cold cooked eggs are the Highlight of the Menue spread :-) Both Lounges are located above the Starbucks.

those lounges as well as the whole AB is a one big joke. poor selection, old tired interor, you're better off in any restaurant on the airport and this does not compare anyhow to LH/* lounge.

Was there couple of weeks ago.
Was told to use BA lounge as it was supposedly better than AF lounge.
(tough to believe given condition of BA lounge)

Plus point for BA lounge: Staff is nice

Bad points: everything else (although it ws exactly lunch time - noon - \, there was hardly anything to eat (especially for a vegeterian - not even a cookie or banana or apple).

Also forget about showerrs, this one does not even have rest-room!!

(explanation given was that this entire airport was supposedly been shut down for few years now)

Exactly the point. Don't expect anything spectacular as the whole airport will be closed as soon as they finish BER.This has been delayed several times and is now expected to happen sometime in 2014 (or maybe end 2013 as suggested by some sources last week).

Technically, the designated AB lounge at TXL is the AF lounge in Terminal A. However, as a OW status passenger, you can also get access to the BA lounge on the floor below. The space for both lounges is exactly the same, so are the offerings. I would have to give a slight edge to the BA lounge. One thing to keep in mind though - if you visit one of the lounges, you won't be admitted to the other. It happened to me 2 weeks ago. I had visited the BA lounge on a prior connection through TXL, so decided to go to the AF lounge first to check it out. I got in, showed my credentials and my AB boarding pass, they stamped it and let me in. I had a 4-hour layover, so after a while I decided to go down to the BA lounge but they refused to let me in as they claimed that AB won't pay for a 2nd lounge (they saw the stamp on my BP). So, be aware of that. Odd thing is that same morning I used both the international lounge and the Schengen one at DUS with no problem. AB have weird access policies sometimes. Again, I would recommend the BA lounge over AF.

What if you were using a mobile boarding pass? Hard to enforce no visiting both lounges.

mobile BP is ok, i used both few times

The BA lounge usually has a nice (though never changing) selection of sandwiches.

only hot food item at breakfast is toast

Quick question: i am regularly flying domestic with AB from Berlin and holding Topbonus Silver Card. Am i entitled to use AF or BA lounge, as well (this exclusive AB area has only access once leaving from gates A1 and A2). Thanks a lot

Silver gives no Lounge access. Only the exclusive waiting areas.