I actually reread this only a year ago, and I was planning to move on to the next book in the series, The Dark is Rising, when my library got both books on CD. The temptation was too great and I went back to the beginning again to listen to Over Sea, Under Stone before moving on to the second book.
I admit that I don't think this is the strongest book in the series. It's a good place to start - more of a children's adventure/treasure hunt than the detailed exploration of good vs. evil and deeper powers that the later books become. But it's a great story all the same as Simon, Jane and Barney spend their summer holiday searching for the Holy Grail in Cornwall. It's a well-written, well-told story that stands the test of time nicely. Yes, you can tell it isn't written about modern children, but that just helps set it into its own place in history rather than making it either old-fashioned or unreadable.
By listening to the audiobook (very nicely narrated by Alex Jennings), it took me a lot longer to get through than it would have done if I was reading the book. That made the story a little disjointed for me and I lowered the grade for that, but the story itself remains excellent and I highly recommend both Over Sea, Under Stone (I just love that title) and the entrie Dark is Rising sequence to any reader.
Over Sea, Under Stone
The Dark is Rising, Book 1