I've seen many Cathedrals in Spain, and this was probably my least favorite. From the outside, it is very impressive, and makes for cool photos. But from the inside, I found it a bit depressing, with its stark white walls and stark gold altar, and a complete lack of natural OR artificial light. It did not inspire me with peace or any positive feelings at all, on the contrary. Plus, the bathroom facilities were horrible, dirty and no soap or paper to be had.
We were hoping to access the Capilla Real (Royal Chapel) with the same ticket (4 euros), but were disappointed to find that it had its own entry charge, also 4 euros. So we skipped that.
The area around the Cathedral is very interesting, street musicians and vendors and lots of narrow alleys filled with crafts and shops, but I don't think it's worth paying the entrance to go inside, unless you have strong Catholic inclinations.