Their IPv6 situation was even worse. They used 6rd and I swear, the translation box was probably a single router or Linux box with a 100 Mbit NIC in a rack somewhere. If you bothered enabling 6rd, every v6 site would be awfully slow. Even the browser projects to automate the selection of v6/v4 didn't help.
When I finally moved away and cancelled the service, I mailed my modem back as directed. A few months later, they sent my account to a collections agency over the cost of a modem, which their system claimed to have not received. I spent hours on endless phone calls but ended up just paying them the $250 or whatever to save my credit and stop the madness.
Seriously, they were the worst provider I ever had.