That album changed the way 'pop' music was thought about, more than any that had gone before it and after it. It's hard to think of any other album that changed the way music was created or thought about. It also broke boundaries as to how far you could push pop music.
You have to remember that up until, say, the mid-60s bands like The Beatles etc were thought of as what we'd think of as boybands or similar today. It was music for kids. It wasn't an art form, just disposable music. This album pretty much changes all that - pop music is now a 'serious' art form. It also shows every other musician what you can do and pushes the idea that you can use Victorian fairs, drugs, avant-garde classical tape loops, Indian influences, classical orchestras, psychedelics, childhood memories and blend them into rock and make it look as natural as a nursery rhyme.
You have to remember that about a year beforehand they gave up playing live as they couldn't hear themselves play. They then decided to make an album in the studio and would use it's full potential. It had never been done before so no one had any idea as to what a studio-only album could sound like. When it was released, it was jaw-dropping. There's no comparison that you could make to it today.
The album cover, printing of lyrics on the inside and how it was recorded was unlike anything you'd seen or heard before. When it was released, it was an event and it's difficult to imagine what it must've been like to todays ears. Bear in mind Strawberry Fields and Penny Lane were to be on the album but had to be released as a double-a side to show they were still going.
You may not like it, but it is definitely not overrated. It is probably the most important album in rock music history.