Maybe I just have had insanely good luck. And I'm sure a lot depends on where you are. But at three different locations I've had GCI internet service since 2005 (Anchorage, Palmer, and then Wasilla) I have nearly always got the advertised speeds. I do regular tests on my 100 mbps connection these days and consistently get sustained 100 mbps +/- 1 mbps at speedtest.net. 

"Laughably" small bandwidth caps start at 50 GB/month and go all the way up to 1 TB/month, on par with what you'd get in the lower 48., albeit at probably twice the price for the same bandwidth.

As for slowing down to dial-up speeds, not true. They will slow you to 768 kbps after you reach the cap. Dial-up tops out at 56kbps, so that is still 13x faster and not a bad connection for basic internet surfing.