I have 6 Billys and my parents have something like 10. When I worked at a small startup and needed to fit out some field offices, I bought two things from the store, a Galant desk and a Billy bookshelf for each employee. Two Galants turn into a reasonable conference table. The employees loved both, a simple sturdy desk that doesn't dictate function and a highly customizable wall, they quickly became filled with all kinds of personalization. Cost per employee? something like $250.
They lasted for years and when the startup finally sold and closed up, the new owners didn't want the furniture, so we asked around and found a battered women's shelter who had just placed a woman and three kids into an apartment. They had been living without any furniture at all. Suddenly our reception couch turned into a living room couch, our Galants turned into their dining room table and our extra chairs turned into their dining room tables. Desk lamps became living room lamps and torches lit the bedrooms. Billys became impromptu wardrobes in all the bedrooms. After the move, all they needed were some mattresses, but they at least had an apartment full of other furniture. But I came away impressed at the general versatility of the furniture. Nearly a decade later, that family still has their Billys.
Billys basically get out of the way and turn into walls with storage. I've never bought the accessories (doors, etc.), but all the ones I know turn into curio cabinets as much as bookshelves. Nobody really cares much about what shelves look like, they care about what's on them and in that sense IKEA nailed it.
I also have a handful of other shelves from various manufacturers and it's very weird how they get things wrong and how optimized a piece of furniture the Billy is. It's weird to get so passionate about such a plain looking piece of furniture, but it's really kind of perfection of form.