What does Billtrust do well? Billtrust makes it easy for customers to log in and see their invoices, pay their invoices, and maintain a payment history with you. Although not the cleanest view, the admin portal to manage your customers is very powerful and allows you to help customers get what they need done. You can run powerful reporting.
What does Billtrust not do well? Billtrust is lacking in functional UX and is not "pretty." There are roundabout ways to complete certain tasks, but not many things are straight-forward or easy to use. The learning curve for a non-tech-savvy user is pretty high, but functionality does work.
What do you wish you had known about Billtrust before using? Billtrust is not very friendly outside of its core products, which I consider to be the user portal and the admin portal. Even then, these portals are lacking in efficient design and are hard to navigate. Billtrust's API is not included in the main product and must be purchased as an add-on. If integrations were natively included in the product, and Billtrust allowed itself to be used primarily as a backend processor, it'd be a great product for that.
Have you used a competitor to Billtrust? If so, how does it compare? I have not used a direct competitor to Billtrust. However, I've been involved in building in-house payment pages that did what Billtrust does.