4 year old DS has had his about a year, and uses it at bedtime (after we read to him, brush his teeth, etc.) for up to 30 minutes almost every night. He likes to be able to "read" himself, but still definately prefers us to read to him.
I will say that it doesn't really seem to be teaching him anything, just is an interactive toy.
I'd recommend it nonetheless - and agree with PP - a new book or two makes it a wonderful toy for long trips!