They have a good variety of fields types available and a very strong Salesforce integration.
Sometimes editing the form can be a little wonky in that it might not save correctly and dragging fields around sometimes can be hard. The UI in the Salesforce integration page can be a little hard to manage when you're creating more than one or two records.
Just do the trial and see if it works.
I use Formstack internally and within integrated solutions for our clients at Octiv.
Here are a couple examples:
1. A sales rep is at a trade show to close business (not just market or generate leads) and they need both a document generated with all the correct information along with all of the correct records in Salesforce created with the data. We were able to design a solution using Formstack and Octiv to give the rep a very simple form to fill out, generate a complex document with that data and then push the data to Salesforce. In this interaction, an Opportunity, it's parent Account, four Contact records linked to the Account and the correlating OpportunityContactRoles records are all created at the same time.
2. Using an Octiv document is a great way to deliver a trackable, legal document to your customer, but what if you need to collect information from your customer, rather than only delivering information. Using Formstack to create a form, you can embed your form within your Octiv document to collect that information.
This could be billing information, interest related information, or really anything else.