I really like this series.

Anybody else has the impression that the last 5 or 6 books were not up to the old standard, esp. the stories getting more and more muddled, that is, hard to follow and understand?

In book 6, IIRC, when the Vineans are taking yoko and the others to their homeplanet in their spaceships, yoko asks how FTL-travel is possible. The answer is "What if there is no light?" and then its explained that a "tube of darkness" is created between Vinea and earth, through which the ships travel. Of course, that explanation makes me laugh, but its so sweet and funny, in a way.