That said, it often took forever over USB, so the fast transfer speeds of FireWire was a big selling point.
Wikipedia's iTunes history [1] says iPod support came in October 2001 (i.e. when the iPod was released), and Rio One support in December 2001, and doesn't really mention other MP3 players.
At any rate, the faithfulness with which the iPod presented the iTunes music library was great.
[1] https://en.wikipedia.org/wiki/History_of_iTunes#iTunes_2
"download songs to popular MP3 players from Rio and Creative Labs with plug-and-play simplicity with no extra software or complicated driver installations required."
https://www.apple.com/pr/library/2001/01/09Apple-Introduces-...
Technically, this wasn't any more legal than recording a CD to a cassette tape. It's still an infringing copy. It's just that nobody was ever sued for it, as far as I'm aware, and it's since been legalized in some places.
There were a few CDs with anti-rip technology (including the famous Sony rootkit CDs), but the industry largely gave up on that after losing the DRM war.
If workable DRM had been deployed fast enough, neither Napster nor the iPod would have existed. That's part of why the fight against DRM is important.
But Apple was actually marketing iTunes with the slogan "Rip. Mix. Burn." and put out this awesome ad featuring De La Soul, Iggy Pop, Barry White, and others. Check it out:
I think it is important to remember that when the iPod came out, iTunes already existed. Not the iTunes store, mind you, but iTunes as a solution to (legally) rip your existing CDs (and automatically get the tags/meta-information from OCDDB or so), and manage songs downloaded from Napster (less legally).
So, people had a good platform to listen to music on the desktop, nicely managed with lots of meta-information (tags, time last played, rankings, etc).
Now, with the iPod, you could JUST PLUG IT IN, and it would download the whole shebang within a couple of minutes (via Firewire), and offer it in a nice interface. THAT was a quantum leap in usability compared with all other devices on the market at the time.
I had a minidisk player, and you could add artist and song titles manually on the device (selecting a letter at a time with a left/right cursor), and my, what a pain it was.
Just having your whole (carefully curated) music library on the go in a a few minutes on the iPod was AWESOME.