Ask HN: What do you think of web-based vulnerability scanners?

Our small (but growing) startup thinks we've grown to a stage where we could start becoming a target. Rather than using tools like MetaSploit it seems a lot simpler to outsource this work to a 3rd-party solution like TinFoil Security, ArmorHub or perhaps McAfee Vulnerability Assessment SaaS. Has anyone used any of these or similar services? How do they compare? How effective are they? Any alternative suggestions?