I had a pathetic experience with an ISP "reselling" Bell's DSL service. The ISP's name is Distributel. YouTube would not, at "peak times" (6pm - 9pm on weekdays, most times on weekends), work at 1080. The download speed forced you to wait for the entire clip to pre-buffer before it would be possible to play it.
What was frustrating was that opening an SSH proxy to a US server (hosted at Linode) and going through that proxy would allow you to watch 1080 clips with plenty of pre-buffering headroom. I tried to explain this to tech support (with bandwidth measurements, etc) to no avail.
I finally switched over to TekSavvy who have always had their shit down. They truly rock (the tech support is amazing).