I'd say it's fair to call the transit situation at Tegel inconvenient. You have to figure out what bus to get on, buy a ticket, haul your luggage onto a bus, haul it off, get off the bus, then walk outside and down to the subway platform. This takes 20 minutes and is uncomfortable. If the U7 went to Tegel as was planned, you'd just take the escalator to the subway platform and get on in the direction towards the city.

If you take a taxi or rent a car to Tegel, it's extremely convenient.