In terms of absolute sales I guess the ATV was unsuccessful largely because people couldn't see it for what it was and always wanted it to be a full media centre PC. The fact is, in my AV system I have a TiVo, Blu ray, Xbox 360 and ATV and the ATV gets by far the most use. I've encoded all my DVDs for it and stuck them in iTunes on an old Mac with a 1.5TB drive attached so we're never short of something to watch.
The instant access beats the heck out of DVDs and the simple UI makes for a very neat device perfectly designed for what it does and even my three year old son can use it. I'm sorely tempted to get one of the new ones for a second room since most of my material is streamed but I'll keep the original if only for its ability to buy movies directly from iTunes.
Everyone I've shown it to has been amazed but then the same is true of my TiVo and that failed in the UK market too sadly.