I've been using the workflow system for a few months now and I would imagine that it would be ideal for doing something like an ecommerce module if you define the whole process as a state diagram taking into account all possible situations.
If you want to handle things like when a user should be charged, you could just use the workflows attibutes and test this attribute at each stage where payment would be due.
Alternatively, you could code your pl/sql callbacks to check other tables like ec_orders before doing something like a credit-card transaction and set that table with the stage where transaction should be performed when the user is creating the order.
Hope this helps.
