I always thought most of his books went on a few chapters too long. For instance, I loved Stranger in a Strange land throughout all the "teaching Mike to be Human" stuff, but once he went off to found his own religion it lost me.
Same thing with Number of the Beast - an interesting exploration of the parallel universes idea, but once he got to a point where he was having parties with all the folks from different universes... it was like he'd played the original idea out and was casting around for something to make the book longer because he was paid by the word or something.
Plus, I find his characters tend to be fairly samey. He seems to think there are only two types of person - the "good" type who is libertarian, independant minded, largely unconcerned with the rules of society, lives by his wits in some creative field like writing, etc - and the "bad" type who is basically an unthinking leech, usually a government employee or thief of some kind. It gets old after a while.
All that said, I do enjoy most of his books quite a bit, especially some of the more juvenile stuff. I loved Starship Troopers, Have Space Suit, Starman Jones, and quite liked Stranger in a Strange Land, and Time Enough for Love.
