Yes, it was a lucky bunch of Londoners who got to hear the rooftop concert, no doubt. It's the kind of thing you brag about to your grandkids. (For those who don't know what I'm talking about, allow me to point you to this wikipedia entry and have you scroll down slightly.)
"Don't Let Me Down" was left off the Let It Be album for some unfathomable reason (it would have improved it), but at least it was available almost a full year sooner than that album, as the B-side to "Get Back." The song is a simple yet awesome bluesy thing, a song to lean back a little too far in your chair to, if that makes any sense, but it's also some of the most nakedly emotional stuff John had done up to that point in his career. When I hear "Don't Let Me Down," I hear it as a warmup to the vulnerability of his first solo album, Plastic Ono Band, although whereas that album is so raw it can be somewhat cringe-inducing, "Don't Let Me Down" is still a song you can rock out to.
John's vocal is absolutely what makes this; his strained yowls of passion and paranoia just get into your bones. But as usual, what the band is doing is absolutely perfect too-- listen to Paul's bass, with that octave jumping weirdness he's doing, and you realize that Paul's totally the only person who would have come up with that for a song like this. Billy Preston, who has the distinction of being the only outside musician the Beatles ever bothered to credit on an album, seems to earn the credit here, especially on the bridge, where the paranoia seems to abate a little-- "don't you know it's gonna last" and all of that-- and Billy's keyboard grounds the band. I also love what Billy and Ringo do on the verses, how they play those three note figures that obscure the rhythm. That, and the 5-beat measure the band throws in for John at the beginning of the verses-- man, these are the flourishes of genius that push this beyond the standard 3 chord jam. Of COURSE. Just, wow. Love it.
So, sorry I was late to the rooftop concert, but at least we're here now. Watch the video again-- look how much fun they're having playing together-- and this is the Get Back sessions we're talking about! Awesome.
"Don't Let Me Down," released in the U.K. as B-side to "Get Back," April 11, 1969; in the U.S. as B-side to "Get Back," May 5, 1969.
I am indebted for all discography information to the tremendous Beatles-Discography.com.