Many of us that were impressed with his earlier works feel that way, myself included, mostly- but there were some exceptions. The one that sticks out in my mind is the "Dora's Song" part of "Time Enough for Love". The book as a whole is very much in his later style, but "Dora's Song" could almost stand on it's own as an example of the earlier style.

Unfortunately, it is my undertanding that some of the changes were due to health problems- blood supply to the brain is not a trivial thing.

While there are some of his later works I appreciate, I have to admit I'm still fondest of his "juveniles". His teenage characters are more intelligent and competent than 99 percent of the adult role models this society offers now, and they are striving to become even more competent in preparation for adulthood. If more "juvenile" writing was like this, we'd have a lot fewer problem adults now.