When people say Divi, many times they don't realize they are referring to 2 separate things. The Divi Theme & the Divi Builder. While you would only use the Divi Theme with the Divi Builder, you certainly can use the Divi Builder with any WordPress theme.
First, the Divi Theme is nice, but I find it basic. If I was to build a website using the Divi Builder, I would use a more modern theme.
The Divi Builder is the most advanced page builder that I have ever used for WordPress. It has more options, more flexibility then all the others combined. In practical usage, I think its completely overkill and overwhelming at first, in a bad way. I think they want to accommodate their users and in this have given too many options versus making some decisions for their users. So just be prepared.
Divi Builder is a shortcode based builder. Most people already know this, I don't like it, but you will have to decide for yourself whether that is a big deal or not. My issue with it is Elegant Themes could easily solve the problem, but they haven't.
As far as templates go, there are a TON of them, again more then all the other page builders combined. They keep making them because they sell, but I don't find them that practical or useful. Sure they are beautiful, but they are more for learning purposes versus practical usage.
Right now, you can't build custom headers or footers with Divi. You can't build custom archive pages or page templates. Divi has said that is coming and I am pretty sure they are motivated to deliver. Just know its not there now.