The ease of use, the ease in which you can add new fields, the speed at which you can add your own custom fields or even change the layout.
I would like to have a little more control over how the field appears, but it's easy enough to make minor changes. Also, the ability to create multiple versions of the same form is handy when there are different versions of an item (item 1, item 2 etc). And of course, the fact that you're using a cloud-based solution for all your forms makes sending them out to clients/customers so much easier than having to send PDFs by email.