'Only' 64k and 128k speeds. However, it was 'always on' compared to dialup over POTS lines. 4-5x faster for uploads and 2-4 times faster for downloads, so most dial-up users would have coveted it.

and the monthly cost kept it out of greater adoption.

I believe it was also used for high fidelity, low latency audio for interviews and music production.

Don't forget video games.

In a game with good network code such as Quake 2 / 3, you could expect at best a 150ms ping time with a 56k modem in ideal scenarios but on ISDN you could get half that which made a world of a difference.

I never had ISDN but I knew a few people with it. It was like you were playing a completely different game. Then cable came and it blew everything away with getting 20-30ms ping times even in the early 2000s.

Ah, I was only playing original Quake with a direct dial-up to a friend’s modem, for 1-on-1, but we did have it working in the computer lab on 10-baseT.

If somebody went to turn their desktop slightly, the coax would pop out of the t-connector and those further down the chain would be at a severe disadvantage.