They at least go into far more detail and flesh the characters out much better. I enjoy a lot of novelizations simply for the added scenes so I might be a bit biased.

TPM works better but is only a decent read. AOTC is pretty much the same. Both are well written books but share the limitations of the films.

But ROTS...that's a winner. Not only does it wipe the floor with the film, it makes many Prequel-isms palatable. This is due to the writer, Matthew Stover, who also wrote the phenomenally dark Apocalypse Now-esque Shatterpoint.