I read all of them this year.
The first three are a trilogy, and work well together.
The 4th is pretty much a bridge to the last planned trilogy. It's very heavy on philosophy, in a much different way. A lot of folks think it's the best book of the series, but I'm not sure I agree with that. I like my science fiction a little less philosophical, I guess.
The last two are good, if much different. I wound up liking them a lot, but they are set in an almost entirely different universe than the first trilogy (it's the same universe, but many thousands of years in the future). There's some weird sexual undertones to them, but the sci-fi world building is off the charts. You can tell it was set up for a third book to finish it off, but Frank died.
I beleive his kid and another author "finished" the series, but I haven't/don't plan to read those.