Granted, even if you are reading in the original language, you should probably have annotations unless you're comfortable with 13th century Latin. Knowledge of the classical period isn't sufficient.
Why are you reading it?
I am reading it because my friend, a theology(Divinity) major, recommended it after a conversation. My friend is working on his Masters, and is Anglican. I am a Catholic, and, while relatively lax, feel it is important to understand my religion, and the understanding of Christianity as a whole.
He recommended Summa Theologica as a way of understanding Christianity more completely.
However, as I am an accounting major, and have not studied Latin, and am reading a translation, I am having severe difficulties reading the book.