I did have problems about 2 weeks ago. I was on US airways, and both legs of my flight coming home, they gave me trouble about having 3 carry ons (CPAP, purse, backpack). I'm young (26) and look younger than that, so when I explained that one was medical equipment, they kind of looked at me skeptically and eventually let me pass. This was at the San Jose and Phoenix airports.
When I was going through security in St. Louis, the agent that was checking my CPAP actually dropped it. I was worried it wasn't going to work, but ended up being fine.
I never tried to use it in flight though,