Personally, I love the first four books, although the be warned they take a different tone than from the first book. The last two are good, but Heinlein passed away after writing the sixth book. I believe his son tried to continue the series, but I haven't read past that.
I highly recommend reading at least the first four books.
I love them all for different reasons. The implications of the Scattering in the latter books is pretty mind bending to me, few sci-fi books went that far.
