What do you like best?
Betty Blocks offers all elements needed to create custom web applications in a nice looking and very easy to use interface. Apart from the basic aspects (datamodel, screens) it is the additional possibilities (workflows, actions, external interfaces, rights, expressions) that really make this platform very powerful.
What do you dislike?
Not much. A better way to create 1:n relations would be nice, and build-in multi-language support (but that is coming soon). Overall, most wishes are added quickly.
What business problems are you solving with the product? What benefits have you realized?
Whenever the system you are building is mostly about registering (and viewing/editing) data, Betty Blocks is a perfect solution. The main benefits are the short development time, and the flexibility that remains when the system is in use.