If you're not in the Apple ecosystem, buying a Homepod doesn't make sense. If you own an Apple device but do not have an Apple Music subscription, you can use practically all the features of the Homepod. Of course, it's a pity that you can't easily operate Spotify via voice control, but have to add the Homepod as an external device.

Either way, I think it makes much less sense to own a Homepod without Apple Music, especially if you want to use it mainly for listening to music. Otherwise, if you're interested in smart speakers, check out Sonos.

You can stream from an Apple iPad, iPod touch, iPhone or laptop via AirPlay (works great). This way you can also use streaming services that are not Apple Music (e.g. Spotify or Amazon or ...).
But the HomePod only really makes sense with Apple Music. With it, music can be streamed directly (Siri can do it) via the HomePod - and the sound is simply great.