I avoided these books for a long time. Why would I ever want to read kids books? But after enough badgering by friends I finally read the first one. I loved it. I had truly been missing out on some good stories. I immediately read the rest of the books as well, and eagerly await the 5th. Its not as serious as a lot of fantasy, in fact its VERY humorous, but its also VERY good. I'm glad I broke down and read them!
You should just give the first book a try. Its very short, and you could probably read it in a couple days. If you don't like it you wouldn't have lost much, but if you do like it, as I suspect you will (as a fantasy fan), you'll be hooked like the rest of us.
-Marshall