Do not, under any circumstances, make this your first Robert Heinlein book. Don't make it your second or third, either. (And don't make it your _last_.)
Heinlein wrote this book right after recovering from a carotid bypass. Those of us who had been reading his stuff for a while were thrilled to see it (I remember lapping it up when it was serialized in _Omni_ magazine), largely because it meant he hadn't been permanently rendered unable to write.
And there's certainly stuff here for Heinlein readers to appreciate. Some readers don't like Heinlein's dialogue, but I like it just fine and I enjoy the interplay among the four main characters in this one. (Nor do I have any trouble telling which of the characters is narrating at which point.)
This is also the novel in which Heinlein sets up the concept of the World-As-Myth. Apparently tired of listening to his characters invite one another to 'have a go at solipsism', he finally has a go at it himself -- and comes up with a 'multiperson' version of it, in which various 'real' universes are 'fictional' relative to one another, yet accessible via six-dimensional rotation using a nifty device invented by protagonist Jake Burroughs. (At the very least, this clever trick allows Heinlein to bring together lots of his characters from his various fictional worlds and let them all have free-love open relationships with each other.)
The downside is that it's somewhat self-indulgent. First we visit some of the fictional worlds created by several of Heinlein's own favorite writers. On top of that, the name of every one of the 'bad guys' is an anagram of some variant of Heinlein's own name, or Virginia's, or one of his several early noms de plume. Then, in a very confusing ending, we're sort of given to understand, more or less, that all of them are Heinlein himself, somehow, maybe. My, what a powerful fabulist he must therefore be.
Back to the plus side. Readers of _Time Enough For Love_ -- those who liked it, anyway -- will cheer the return of Lazarus Long, as this novel not only brings him back (together with some new members of the Long family) but sets up two further novels in which he appears (_The Cat Who Walks Through Walls_ and _To Sail Beyond the Sunset_; don't start with _those_ either). Of course this is a plus only for those of us who _did_ like _TEFL_; those who didn't won't care for this book either.
Interesting late-period Heinlein, then, filled with what Heinlein fans will regard as great characters and great character interaction -- but somewhat bloated with some stuff that doesn't make very good sense and shot through with some extremely trivial intellectual puzzles. (Most of the anagrams aren't very hard; even the one or two comparatively difficult ones won't pose major problems for anyone who knows anything about Heinlein's [and Ginny's] naval service.) The casual Heinlein reader probably won't like it and won't grok it.
It's not my favorite either, but I don't think Heinlein wrote any _bad_ fiction. (His nonfiction is another story.) He _was_ a powerful fabulist, and I don't mind indulging him while he celebrates the return of his power in this novel.