Since I finished reading the LoTR for the first time just days ago I can only comment on one experience. I'll take a deep breath and tell you about it with the hope that I can add something to the discussion.
I'm a bit embarrased to say that I'm 38 years old and just read the book. I read it in less than two weeks while I was bedridden with illness and bored out of my mind. I can't find the words to adequately describe what it was like for me to read the story. I rarely have the opportunity to sit still for any length of time so it was a bit surreal to go from a very fast-paced life with little opportunity for leisure to suddenly come to a screeching halt with a very heavy book in my lap (I have the hard-cover, one-volume edition). I was astonished by what I found once I opened the door that I always knew was there (who hasn't heard of the LoTR?) but never bothered opening. I thought I had too many other "important" things to do than read a fantasy book. Little did I know what a good time investment it would be for the condition of my soul. Reading the story as an adult has reminded me of what it was like before I became somewhat cynical about human nature and of life itself. It's reintroduced a sense of hope and reminded me of the importance of love and friendship. In a nutshell, it breathed new life into my weary bones and mind.
Anyway, I obviously read the book straight through. I very much want to read it again because the material is dense with detail and I know I didn't absorb it all. To be honest, I hope that my urgency to learn more about Tolkien decreases a bit because I'm losing too much sleep reading on the Internet!
I believe my interest will continue... just hoping for a more managable intensity level.