How is this different from a 180 BPM playlist or an app like RockMyRun?

A playlist matches a tempo. A Beatpace mix matches your training. Three differences: the BPM is actually right, because tracks are time-stretched to the exact target rather than tagged and hoped for; each block is the exact length you asked for, so four minutes hard is four minutes, not three forty-two because that was the song length; and the music shifts when your block shifts, with no drift, no half-tempo bridges, no breakdowns mid-threshold on top of that it runs natively on the watch, which the cadence apps do not.

Do I need to carry my phone, and which Garmins work?

No phone needed on a supported Garmin: sync the mix over Wi-Fi and leave the phone at home. Supported watches are covered above; the rule of thumb is that if it runs Spotify or Amazon Music, it runs Beatpace. Prefer to carry your phone? Every mix also runs in any browser with the screen locked, no watch required.

It is AI-generated music. Will it actually sound good enough to run to?

That is the right question to ask, and the honest answer is to listen first. The tracks are original, built specifically to hold a steady beat and shift energy on cue, which is exactly what a structured run needs and what recognisable hits often fail at when you stretch them to a target BPM. They are not going to give you the rush of your favourite song dropping at the right moment. There are samples on the runner page and a 14-day trial, so the cost of finding out is zero.