Having read through all the comments, I find they mostly fall into four categories:
1) People who liked the movies but thought the book was better;
2) People who have mixed feelings about the book and prefer The Hobbit;
3) People who liked the book so much they can't imagine anyone not liking it (using implicit phrasing like, "If you haven't read it, you're denying yourself a treat");
4) People who hated it so much they, equally implicitly, can't imagine anyone liking it. "Slow" and "boring" are their words of criticism.
Only one of the last group attempts to explain how the book became so popular if it's so bad. This person suggests it hit the hippie moral ethos (having been around for decades, plural, before that without being much noticed: not true on either account). I'd like to see him try to explain, say, Theoden and Eowyn on that basis.
What most interests me is that the bashers recommend, in its place, works that _I_ found slow and boring. "I bet many Tolkien fans haven't read most of these," says one, and he's right, I haven't, because the ones I have read were so bloody bad.
JRRT himself had something to say about this:
"Some who have read the book, or at any rate have reviewed it, have found it boring, absurd, or contemptible; and I have no cause to complain, since I have similar opinions of their works, or of the kinds of writing that they evidently prefer."