Upload & Sell: Off
ModelMayhem is definitely a mixed bag for hiring someone you've never worked with before, but it's a great way to build relationships with models, makeup artists, hair stylists, etc over time. You learn who's reliable and you work with them more and more.
People who flake on you (and people *will* flake on you) don't get a second shot and have thus filtered themselves out. That works fine. But I would never hire someone for a commercial gig, having never met them, through MM. I'd contact agencies and ask for experienced models. Hell, I'd ask to test a few ahead of time and hire one afterwards.