When you launch Music Memos, you're met with a very clean, straightforward interface with a big record button right in the middle. There's not a settings menu to be found anywhere, nor will you be doing much swiping around. The only other "sections" to the app are an instrument tuner, located at the top right, and a list of your existing recordings (or "ideas" as Apple calls them). You get to those by swiping up on the main screen or tapping the folder icon.
You don't really even need to hit the record button to get started; Music Memos includes an "auto" mode that will begin recording once it detects you're playing something — or generally making noise. I still prefer the tried and true button, personally. Once you're recording, the button pulsates red and a waveform scrolls along the bottom of the app.
When the take's done, you'll see the recording appear at the bottom. Music Memos automatically splits everything into measures and attempts to figure out what chords you were playing. It does an okay job without any help, but you can dive in to change the chords or pick a different time signature. And if you were using a capo or had your guitar in another tuning, there's a way to add notes so you'll remember those things later on.
From there, you can just hit record again and start on another idea. (There's no way to layer tracks or dub in more guitar work; that's all GarageBand territory.) Or you can add some backing instruments to what you've already laid down. Adding drums or bass requires just a tap, and Music Memos lets you customize things like volume and the complexity of the drumbeat or bass line. In each case, particularly with the bass, Apple does a nice job of keeping in time with your recording — without requiring any real work on your part. It's nothing like having a real person behind you, but still does the job of filling out your song easily and quickly.
Once you're satisfied, you can send your recording over to GarageBand for more polish, or you can skip that part and just upload what you've got to SoundCloud, YouTube, or other services. Again, the underlying principle through all of this is ease of use. Apple never strays from that.
And that's really all you need to know to start using Music Memos. It's simple by design, and there are surely many apps like it already in the App Store. But this one is free, and it's made by Apple. Those two things alone guarantee that plenty of musicians will at least be giving it a try over the next few days and weeks. Maybe it'll be too basic for what you need — in that case there's GarageBand or third-party options. And remember that Voice Memos isn't going anywhere; it's still there if you need it in a pinch. But Music Memos successfully manages to be something different that fits in between both of them. If you're trying to record a song idea on your iPhone, this is what you'll want to use from now on. Hit record, add some instruments to get an idea of what it'd sound like with a band, and done.