WS-Form has a lot going for it in terms of features. I tried out a number of its competitors, and I can tell you that WS-Form is by far the easiest to use. Everything just works. Well, in my case, almost everything. I had a problem with exporting my form data and had to contact support. It is here that choosing WS Form again turned out to be a great decision. Mark Westguard resolved the issue in a few hours.
Summary: Strongly recommended.