I just adored the library. I thought the decor matched the feel of the show perfectly for the early seasons and really contributed to the overall ambience of the show.
I loved that it felt spacious, almost like a nice feeling of Buffy having all the space in the world, yet still confined to her duties in the library.
I also liked that it had the cage, all the reading material, yet was easily accessible to the rest of their life.
I liked the magic box, but it didn't feel the same as it did feel like the scoobs (apart from Anya and Giles) had to really go out of their way to go there. I'm not sure if I'm expressing it correctly, but it just felt a little... out of the way.
The library was a nice inbetween of their real lives (school) while still not being in their homes.