I just did this with the Android app, including the new feature to handle multiple BPs.

I had printed the boarding passes last night, but noticed this morning that they were available in the United app (4 of us on 2 PNRs), so I decided to try the mobile ones at security. Worked fine, but it was a bit awkward having to flip through them all and scan each one. In the end we used the paper BPs for boarding, as it's easier to have everyone hold their own. I'm a techie guy and I really want mobile BPs to be more convenient, but they always end up being less so for me. If I don't have access to a printer, sure, but otherwise I'll continue to print them.