Neither of the Delta mobile sites have worked for me for months, which is really annoying because their main web site takes forever to load on my iPhone.

I used to use them for the electronic boarding pass when going through security, etc. in ATL, but I finally gave up after it didn't work for a few months.

NW's mobile site has still worked for flights booked through NW, but it doesn't offer electronic boarding passes in ATL, I don't think.