I flew from and to Tegel once or twice per week on average last year. I find Tegel to actually be quite ok. Getting there on the TXL is quite convenient from many places in the city, and most importantly doesn't take very long (as opposed to Schönefeld/BER, which is rather far out). The interior could use a remodeling, but the layout is convenient because it has a security checkpoint per gate. I'm sure that is more expensive to operate than the single checkpoint per terminal model that is common these days, but it is quite efficient for getting to your gate.