I went from having 25/4 with Comcast and never seeing any issues at all to Greenstreak 6/512kbps. I have noticed the mentioned "congestion" especially in the late afternoon and at times I've had speedtest results of .74Mbps down and a steady uploaded of .64Mbps. The installer who came to my residence was explaining to me the problem and that they were upgrading sometime in the near future and that my area was more than likely to be bought by another telco soon.
I'm willing to ride this out if the upgrade is swift and painless. Though I would be willing to help bring some new TV White Space broadband technology to the area.