Depends on your game style and context!
For a 20k game, the Great Library is an early 6 culture per turn wonder - it's a must!
If you have an SGL, Pyramids are already built, and want an early game break from research can build up your treasury under a Republic while the AI research for you, it's great!
As a crutch to help you with research when you are falling behind - you aren't good enough at trading techs and knowing which techs you can research to get a monopoly on yet.
It's not OP to capture - if you can crack another civ that was strong enough to build the GL's core to capture it, you probably don't need it.
Now if we are talking about building it, gifting it away to a weak civ, and then recapturing it when you see Cavalry Sir Pleb style..... we may be talking OP.