I suppose the idea in itself, a hierarchical tree of subjects isn't too bad, but the execution isn't very good, first of all because of the strange locations of the root and second level views and second because it's not done in a consistent manner. One link in the second level view brought me out the frames without any warning. It's not very pretty either.
Javadocs are built up in the same way. It doesn't look very pretty there either, but for javadocs case it serves a purpose and it's a lot easier to navigate and use with the frames than without.