Abiliy to export / import forms is invaluable! You can keep testing all you want, then move the form to a clean production site.
Ease of use
My first ever attempt at a webform, a really complex one. Intuitive "GUI" popped out ahead of the competition within 15 minutes of testing.
Got my reply on the following day. Clear, concise, straight with a code example, so I solved my problem in the first try.
An example how it everyone should be doing it: lots of info, combined with working examples and answers to other people's questions.