BizTalk workflow engine is fine but another powerful workflow application out there called K2 Workflow -
http://www.k2workflow.com/. It has out of the box integration with InfoPath, ASP.NET, WinForms, SharePoint, BizTalk, CMS, etc. I have done a lot of work with it. It has tended to be a bit easier to work with that BizTalk's current workflow framework.
Using K2, SharePoint and BizTalk you can build a complete framework and rapidly reduce development times. This coupled with InfoPath RAD development, you can created end-to-end solutions in a couple weeks. I have talked about it a couple times on this forum. Feel free to contact me if you have any questions regarding K2.