All I can judge by is personal experiences. My wife loves Windows RT. It plays all of the Flash videos in the sites she visits, she is addicted to that Jet Pack game, and the battery easily lasts all day. She has no idea what ARM is. She understands the loss of backwards compatibilty, but it doesn't bother her. She grokked the charms right away and appreciates that she can always find settings and search in the same place.
I have had no productivity hits using it on my desktop or my wife's old laptop. I'm not usually in the Start Screen for too long on those, but it's great for checking emails or surfing the web. When I'm ready for real "work", I drop to the desktop and use the same apps I used on Windows 7. The only thing I miss is Share charm integration with desktop apps, but I understand why that would be difficult.