Like on the Fitbit Ionic, Versa and Samsung Gear Sport, there are two ways to add tracks to the Garmin -- via third-party music apps or local storage. For now, though, the Forerunner 645 Music supports only iHeartRadio, which has limited options for customizing your playlists.
The alternative is adding your own files to the watch's 3.5GB of onboard storage, and this requires a lot more effort to finagle. You'll first need to download Garmin's desktop app (called Express), then connect your watch to your laptop (via USB). Express scans selected folders for music files, then lets you pick which ones to send to the Forerunner. This method is similar to Fitbit's and can be tedious if your music library isn't well organized.
To my surprise, on my first attempt, only two of the ten songs I picked made it to my wrist, and they were both .m4a files (the others were converted MP3s). That's likely an error resulting from how I converted the songs, but it would have been nice if Garmin Express let me know that the tracks didn't sync. The company says MP3 and AAC files are supported (.m4a is a type of AAC file).
With your songs loaded, you'll need to pair your headphones. Connecting your cans for the first time is relatively simple. Scroll to the music player on the watch; it will then prompt you to pair your headphones. In three steps, I had my OnePlus Bullets Wireless connected. Strangely, because the Bullets were synced to my phone before that and I was streaming a YouTube video to my Chromecast, I somehow ended up controlling my TV with the Forerunner 645. Granted, this is a very specific scenario, but it's a curious one nonetheless.
When the Forerunner 645 works like it's supposed to, it offers robust music controls. You can play, pause and skip tracks (of course), or choose different sources and organize your playlists. I didn't notice any skips when I played my songs while walking around outdoors, but a colleague noted some interruptions in the connection when he turned his wrist away from his head. That's irritating -- you'll have to deal with the potential disruptions and pauses in your music when you're out, unless you can ensure that your watch is constantly pointing in the right direction (or in a more enclosed space).
Another annoyance involves switching your headphones between watch and phone. When I unpaired my earbuds from the watch and re-linked them to my Pixel 2 for my commute, I couldn't get the Forerunner to connect to those earbuds again and had to use another set. I had no such problem pairing my colleague's AirPods to the Forerunner and back to his iPhone, so this seems like a limited issue for now. The company hasn't gotten back to me with an explanation yet, either, so it's not clear whether it's just a general Bluetooth bug.
The music might be the new thing here, but the Forerunner 645 is a capable fitness watch in its own right. Its design is a noticeable improvement over older models (like the Forerunner 25 or 35). According to our executive editor Dana Wollman, who uses Garmin watches for her marathon training, the 645 is lighter and smaller than early models, which makes it easy to wear all day long. She also likes the soft, flexible rubberized band.
In addition to the comfortable design, the Forerunner 645 has features like step counting, sleep tracking and smartphone notifications that make it appropriate for daily use. It's not the most stylish watch, though, so you might want to swap it out for something classier for a formal night out. I also rarely had any trouble reading the "transflective memory-in-pixel (MIP)" display, which looks like a more polished alternative to color e-Ink.
As someone who's more accustomed to touchscreen smartwatches, it took me a while to get used to Garmin's OS. Using the five buttons surrounding the Forerunner 645's face to scroll through and find the app I want is tedious at first, but after two days, muscle memory kicked in. This is similar to how most of the company's newer watches work, though, so Garmin fans will feel at home.
That said, I wish it didn't take so many clicks to see my notifications and reply to texts -- you need three pushes up from the watch face, then two pushes on the notification before you get to the preset responses. But at least you can rearrange the order of activities on the workout page so your favorites are easier to reach.