I've just finished S7 of Buffy and I have to say the First Evil was kind of a let down. They made it out to be the source of all evil, but honestly it didn't seem all that threatening at least not when compared to Glory. Even with Caleb acting as vessel for the First's power it didn't seem that strong. I mean Glory was only at a fraction of her full power in S5, but she seemed like she could have taken Caleb out pretty easily. I don't know I just feel like Glory would have made a better final Big Bad than the First.