Personally, I think Friday holds up better than most of his stuff. The juveniles are okay, though I doubt any kids would be interested today; it's just too dated, and too simplistic. Most of the later unifying works like Cat, Number of the Beast, etc. were pretty much garbage when they came out IMO, so there's really nothing to hold up. The middle stuff (Double Star, Moon is a Harsh Mistress, etc.) is probably the best, but even there, it's more of a nostalgia thing than any great quality. Heinlein was a great influence on the field, but...