In my opinion, InfoPath is a great tool for building GUIs and it is built completely on an independant standard (XML, XSL, etc). For workflow, you will have to build some extra code to move the form from one user to another.
I have recently built a purchase approval process for a manufactoring company in 9 weeks using InfoPath for the GUI layer, K2 workflow management for the human to human process, BizTalk for the system to system integration and SharePoint for the work flow entry and task management of the system. We were able to create an-hoc approval and denial process using K2 has a hub in three weeks out of the box. I work for a Microsoft Gold partner and can give you guidance if you need.
Jason
http://www.k2distillery.com/