I have been using themify themes since the past few years and I have never looked at anything else. I had tried a few other builders based on layers etc, however the flexibility given by themify themes, and especially the builder is unparalleled.
The builder tool gives you the flexibility to create your own templates and themes right from scratch, and after you are past the learning curve, it becomes a really versatile tool to use. Especially, like me, you are not a coder yourself. I have designed a websites for myself and clients using either their builder or the ultra theme, which gives you virtually unlimited combinations in design, so that no 2 sites look the same.
Their plugins and extras are also well thought of and available at a small fee, however it makes sense cos you might not use all of them in one site after all.
There are a few quirks though, for eg. They have given the backend and the frontend modes to work. The backend lets you work on a more logical and structured fashion, and the frontend is purely to design and edit your site right on the page itself. However, sometimes, after working on their frontend, there is some sort of a bug or a delay in saving where changes are not registered and you will seem to lose any changes you made, till the last point you saved. Hence I recommend saving your work more frequently.
Their customer service is pretty good as well, as they help non coders like me with small pieces of css codes if you get stuck somewhere and want to get the work done fast. And they respond almost within a few hours. They also have a comprehensive help section on every theme and tool on their website, which has videos to guide you as well.
As for their themes, they keep adding new ones regularly to an already amazing existing collection, which suit every need. And at the prices that they give those sites, it’s a steal.
I have a suggestion for their team though, if they can include a request section on their website where users can request certain plugins or features which might be helpful to everyone in future. If they can create and provide those features on request, there will be nothing like it.