I have a large lined tetsubin and i use it for brewing black tea.
As long as you prewarm it properly it has a very good heat retention, which is why its good for black tea.
I would definitely recommend it for cheap to medium priced black tea. I wouldn't use it for high grade stuff however, since there is definitely some iron/rust taste added (yes even with the lining). Also the lining probably has some small effect on the taste.
But for a big pot of decent quality Ceylon (or whatever black tea you prefer) its pretty good