Even I had a similar experience with their WebSphere Application Server. 3.5 was like a kia(not that I have used one) and 5.0 was a lexus(not that I have driven that, I drive a Honda). Left me wondering how the same company could produce crappy software and then a well designed,documented, easy to use software.
My guess is that WebSphere 3.5 was just stitched together using multiple software from different teams,the get to market quickly strategy and 5.0 was built ground up using a very good team of programmers,build a good quality software strategy.
I would buy the book if someone would care to write a story about WebSphere 3.5 to 5.0.