First, it already has integrations with a basket of popular Internet services and payment gateways, so it eliminates the need to develop for those APIs and maintain those integrations yourself.

Second, for firms that paid an outside developer to build their site, FormSpring forms can be added without the use of an outside developer, which is typically a costly affair. Rather than paying a developer to build and maintain integrations with services and gateways, a firm can just pay a simple monthly fee, which could very well work out to be much cheaper than the up-front sum given to a developer (possibly several thousand dollars). FormStack also gives website owners the ability to create more forms or modify existing ones without outside help, saving them more money as every interaction with a developer results in a billable hours.

I'm not saying any of this because I dislike developers as I am one myself from time to time, I just think that any service that is designed to prevent us from reinventing the wheel, saves website owners development cost, and gives them more control over their own site is a good idea.

I hadn't heard of of Formspring, but after looking I'm not sure I see the point. The webform module for Drupal seems to do the same thing (and possibly more), but is free, and can provide native content, where appropriate, for the customer site itself rather than being held on another 3rd party domain. If an organisation has downloaded and configured Drupal (or had someone do it for them) then adding contrib modules such as webform, and setting up one or more forms, is trivial in comparison.

I haven't compared those applications directly, but I'll try to round up some data on a future post. I've found that simply trying these programs out for yourself is the best way to figure out what works for your organization. It's not always the case that one application is better or has more to offer, but instead that each one has different features and different approaches, some which may work better for you and your particular situation.