They are easy to integrate into your SDLC, they are not going to replace manual testing or secure development practices but they'll help a lot. They'll pick up tons of stuff for free, they'll remind you best practices.
I have a startup (at least it still feels like a startup!) and we are developing a web application security scanner called Netsparker [0]. It found over 100 zero days in open source applications while testing it [1], including very popular vulnerabilities in applications such as Wordpress and Joomla. I guess that by itself proves how good scanning can be.
If you want to try it on your websites and see it for yourself drop an email / message to contact@netsparker.com with a mention of HN and I'll get you a fully functional trial that you can use on your own websites.
[0] Netsparker Cloud https://www.netsparker.com/online-web-application-security-s... - Netsparker Desktop https://www.netsparker.com/web-vulnerability-scanner/