To begin with, I dunno about that whole parody argument. Seems a bit thin to me. The only part that maybe stands up to that kind of reading is the fact that pirates turn out to be completely incompetent on land. They can't ration effectively, they suck at hunting, and they're drunk most of the time. Come to think of it, they're apparently rather useless on the ship too, at least this bunch is - they suck at navigation, and seem to have no idea what's going on. They're vicious mercenaries who act completely based on their own self-interest, but they're too dumb to recognize what that is half the time.
The writing is somewhat dry, and the story isn't that great. But as the above paragraph hopefully indicates - the pirates are entertaining anyways. The weakest parts of the text are, basically, all the ones the pirates aren't in - Jim, the Doctor, the Squire, who cares. The Doctor gets some funny lines, but their story isn't that compelling. Long John Silver, on the other hand, is a fascinating character, and more complex than everyone else put together. Ruthless and greedy but also capable of planning and saving money, he leads a pirate's life but also purportedly has set aside a nest-egg to retire on with his wife. Tricksy but generally honest, you find yourself rooting for him no matter which side he's on.
One minor thing that intrigued me were a few throw-away lines about blacks - I'm not all that familiar with Stevenson; was he a big supporter of black causes? Because aside from generally being positive, there are a few moments that seem to go out of their way to say nice things about black people. Which I thought was really neat, but you do kind of wonder what the deal is, because they're pretty clearly tangential insertions. For example, "The sight of all those good-natured faces (especially the blacks) (...) made a cheerful contrast to (etc etc)" (p290 in Google Books) - why especially? You know? It just seems like a kind of added emphasis for no apparent reason.
Is Treasure Island a must-read? Definitely not. In fact, your time is probably better spent watching the Muppets movie version (which, by the by, has a different ending, and a rather less satisfying one, if you ask me). Great as the pirates are, if you really want good pirate fiction, read Sabatini.