What kind of clay are the kyusus on the o-cha site made from? Will they season?

They are made from Tokoname clay I would assume. Some are glazed and will not season like a Yixing will.

Unglazed ones really do not season nearly as much as a Yixing either IMHO. It seems the clay in kyusu is much denser and does not absorb the tea like Yixing. As a result, they are OK for multiple types of tea except more aromatic/flavored/scented types.

My oldest one which is only around 10 years old, but was used very heavily for almost 7 years, seemed to brew teas "sweeter." Mildly so, but it was noticable.

Read from www.hojotea.com they explain about the clay used both with tokoname and banko kyusu`s. Banko kyusu made by famous potters such as Jitsuan or Tachi Masaki are made from purple clay, and they work like yixing purple clay, and tokoname clay "It was found that the clay in Tokoname area was very similar to the one produced in Yi Xing area in terms of mineral compositions. By then, Tokoname developed the refining method of clay and successfully introduced 朱砂 Tea pot. In year 1878, Tokoname invited 金士恒 from Yi Xing. He had a significant contribution to the further development of Tokoname tea pot. The Tokoname tea pot has been improved and its design became very refine thanks to his contribution. " I guess not all kyusu are the same, those glazed ones I would avoid, but the ones made by famous masters are generally from good clay, so I would trust a famous potter to make kyusu`s that are beautiful and functional at the same time, and functional means that it is light weight thin walled, good pouring and made of good clay.