I've used it, personally, to implement a public-facing website. That site endured the dreaded 'slashdot effect' several times. No failures.
When implemented properly, Marklogic is damn near unkillable. It will slow down, it will reject connections when queues are full, but it will not fall over. Naturally, this assumes proper underpinnings and capacity calculations. With Marklogic, those are actually documented.
Mandatory disclosure: I do not have and have never had any association with Marklogic other than a paying customer.