'Cause that's what I've seen so far from Stripe. They're incredibly responsive and helpful. They even changed their SSL cert provider for me because older BlackBerries had a rough time with their prior cert provider.
Not to rag on Braintree, they've sounded like a good choice for a long time, but Stripe's changing some of the rules of the game. Killing the need for a merchant account is a really big deal and I'm happy to pay their rate.
[1] This isn't a rhetorical question, actually. I'd be interested in whether you can do this with Braintree.