> It was ridiculously reliable.
Back in the late 90's and early 2000's, getting broadband was a problem where I lived. I oscillated among a few wireless internet providers (actual 802.11 Wifi to a repeater 11 miles away in one case,) and acoustic modems, as I changed properties.
For a couple years I used Qwest ISDN. That was by far the most reliable and consistent Internet I'd ever seen: it wasn't fast (128 Kbps,) but it never went down, and the latency and jitter was lower then anything I've had, then or since.
ISDN was awesome. I had that going on for a bit, too. It was great to experience parts of what some folks (mostly the French, IIRC) had commonly used for such a long time.
Nearly-instant dialup. And not just for a single ISP, but other ISPs as well: The circuit and the Internet service were provided by different entities.
Switch to a different ISP? No problem -- no appointments or installers making new holes in the house required. Just plug in a different phone number, username, password, and done.
And since each B channel was independent, one could do voice calls while the other did data -- dynamically, as-needed. Performance was resolute: Calls were perfect in their consistency, and data rates were precisely 64 kilobytes per second, per channel, symmetric, and not one bit more nor less -- and with constant latency (what jitter?).
And to not leave it to implication for those who don't know: An ISP wasn't required at all. Two people with ISDN could move data between their computers without involving the Internet. The circuits were switched in an any-to-any to fashion.
Want to play a two-player computer game a buddy, with voice chat, over ISDN in 1999? No problem: Use one B channel for data, the other for voice, and get gaming. The circuits are dedicated to these tasks for the duration of the game, and latency is a fixed constant (no Internet used at all, and no lag spikes either).
We've really lost something with the death of this point-to-point, circuit-switched technology. We're probably better off with the best-effort packet switched IP business we wound up using instead, but we've lost something nonetheless. It offered some neat opportunities and was a fun system to explore.
My ISDN was sold as "ISDL" by an ISP. Still had the performance you're describing, but it was tied to them. There was no dialing on my part: it was just always up. I'd pay for it today if an ISP offered it at a low cost, as a backup.
I missed the IDSL phase completely. I'm not even sure if it was ever available in my neck of the woods.
For me, it the continuum went like this: Dialup > ISDN > dialup > slow DOCSIS > faster VDSL > faster DOCSIS > [this is the part where I write a whole chapter about how there is fast, cheap gigabit fiber available in rural areas directly surrounding my small city, from multiple competing companies, but none within the city limits]
Anyway, IDSL. That technology skipped right by a lot of what was cool about ISDN. For me, real ISDN was always-on unless I disconnected it for some reason. While still "dialup" in the strictest sense, it was not infrequent to have sessions that went for months without any interruption at all. But I could also do anything else I wanted with it.
And backups: Apparently these days, a person can get a slice of Starlink pretty cheap. In this mode ("Standby Mode," IIRC), it provides a slow, always-on connection -- I think it's $5 per month for ~500Kbps.
The RV and snowbird communities hate it because it isn't free (they used to be able to pause service in off-season without monthly cost), but it sounds pretty good as a fixed, domestic backup: 500kbps is a lot more than 0. (And if this backup needs used for a long time or speed is important, then: 500kbps is way more than enough bandwidth to log in and pay for a month of real service.)
Playing TFC, I always got faster ping times than the early cable users. ISDN was great.