A lot of the issues with sales tax are not knowing your regulatory requirements and set up. I'd say that's probably worth $4k, but then going forward you still have to pay them that amount. I'd say it would be more worthwhile to pay an accountant to do that for you, and save the ongoing fee. You will have to pay an accountant anyways to do your tax returns. I'm an accountant and my firm often does sales tax returns for our clients. Now, if you're making $1 million a year, that's $40k you need to pay Stripe for the privilege of not worrying about sales tax, compared to a few thousand you'll pay your accounting firm to do it.
The API and integration options are great and I hope Stripe is successful. Really, if they are, it means I can just charge more for my services.
Edit: As others have pointed out, I'm bad at math. I'm going to leave my shame up here but I realize it's $400, not $4000. Just goes to show you that accountants are just like regular people, and that you shouldn't rely on your accountant doing things off the top of their head not using Excel. The order of magnitude difference really shifts my opinion of it.