I just noticed last night after midnight that they finally added TCM-HD (along with several others like MGM-HD) to my Comcast system. I had seen it in the online program guide several days ago so I figured it must be imminent.
Anyway, I've read the comments above and many of you report that it doesn't look any better than the SD channel. I haven't watched enough to say for certain, but when I turned it on for the first time they were showing the widescreen western "Cheyenne Autumn" and it looked much, much better on the HD (channel 890 here) vs. the SD (channel 64). It may not have been full HD, but to me it looked at least as good as an anamorphic DVD, maybe better.
Then this morning I compared the two channels showing the old B&W 1938 film (4:3, obviously), "Marie Antoinette." This time the differences weren't nearly as great, but the HD channel still looked better to my eyes. The SD was noticeably "softer." By the way, the HD channel lags the SD by a few seconds so it's convenient for quick A/B comparisons.
I'm watching on a 9-year old Pioneer Elite 610HD (58") CRT RPTV, calibrated one year ago this month.
I'll have to watch more to get a better idea, but based on my experience so far I'll opt to watch the HD channel. I can only imagine how nice it will be if/when they get a lot of true HD content.