There are a growing number of Premium WordPress Frameworks, and that makes it a challenge to sort out where to start. A lot of people get hooked on one framework and they become very proficient at using that framework, which I think is great. For custom frameworks, there are two Giants I recommend looking at: StudioPress Genesis Framework and Elegant Themes Divi
AThemes did a great comparison article of Genesis vs Divi (http://athemes.com/reviews/genesis-vs-divi-pros-cons/ )
Why Use a Premium Framework?
In most cases, it has become more efficient to develop a Child Theme of a Premium Framework, than continue re-inventing the wheel with custom WordPress development.
Then, there is the added bonus that these frameworks give you access to excellent premium themes. The Premium themes make a great starting point for some projects, as right off the bat they save a bunch of development time. Now of course not every project is going to start from an existing theme, and then get customized. It all comes down to your own specific project, and whether or not a full custom child theme is right decision.
Another big bonus to using a premium framework; you get connected to excellent communities who can help you resolve bugs, develop new features, and see what is going on with other similar types of developers.
Even though they have a cost to them, they speed up development time, allow you to keep up with industry standards within a community as opposed to on your own, and they offer great support. Why wouldn’t you want to use them?