I think its hard to imagine it being "so terrible" because we simply didn't use the internet like we do now, so the speed wasn't as much of a limiting factor as people who never experienced the joys of dial-up assume it would have been. Online gaming, media streaming, high resolution images on every webpage, etc weren't the norm.56k? I thought I was living in luxury when I had my 14400 modem! Just like pain I think my brain has blocked a fair bit of memory of dial up and I only remember it vaguely. Probably it was all sorted out by the time I could afford one. Back in those days you could buy a pretty good 4wd for the cost of the latest computer system.
edit - yes probably over the top. Lets say a not too bad second hand one.
Also, like everything, it was an evolution.. It wasn't like we had 40mbps connections with everything designed and functioning as it does now and then fell back to 14400kbps. We had 14.4.. then got 28.8... 36.. 56.. then ISDN.. then ADSL... Most of the "early adopters" upgraded with the technology, and the usage was tailored accordingly. We got used to bulletin boards, IRC and websites which were primarily text based with maybe one or two low-res images.. then more images.. flash animation.. more images.. downloading music and movies (over multiple days or weeks at times for a single album or movie haha).
I remember playing Counterstrike on dialup against a mate in the US who had a ~10mbps cable connection. That was fun. I would basically be trying to guess where he would be in a few seconds and firing in that direction, get shot in the meantime and repeat.
Edit... just to add, I know that 14.4 wasn't the earliest.. its just what we had when I got internet at home..