When the Hagia Sophia obsoletes, you get its effect anyway. As a result, whether or not you want to build this wonder depends on your technological path. If you're aiming to get Steam Power sooner, pass on this wonder. If you're aiming to get Steam Power later, the Hagia Sophia may be highly useful. If you lack food, the Hagia Sophia is useful for making up for the lack of Workers you may have as a result.