We were lucky enough to be able to integrate the Stripe Tax beta with Billflow.
In my opinion this is the coolest thing Stripe is launched (For SaaS) since they came out with subscriptions. It just works™.
Implementing Stripe Tax was dead-easy, to get it working we essentially just had to switch a boolean to true on our subscription creation code.
Also, the new functionality of the upcoming invoice API is something we've been wanting for a while - being able to estimate the first invoice for a subscription _without_ the customer existing beforehand makes life so much easier when checkout is concerned.