Depends on who you ask...
I say it's MUCH more useful than the ipad by a galaxy, and much more useful than an android tablet by a long mile. You just don't have third party Desktop apps, so you're left with the metro apps, File Explorer, and the whole Desktop feature set excluding I Storage Spaces and a few other things I believe.
If you're looking for a secondary PC that won't be used for a ton of things, outside of Office 2013, Windows RT is by far the best choice. Can you manage your iphone with an ipad? No. Can you manage an android smartphone on an android tablet? No. Can you manage a Windows Phone or even android smartphones with Windows RT? Yes. Can you use most printers without issue?
It's the simple things like that which makes Windows RT stand out. As I see it, RT is the best ARM based operating platform out there. Some hackers have been able to remove Desktop app restrictions on it, and opens the potential for open sourced Desktop apps on RT. Last I heard of, some Russian hacker claimed to have 80% of Win32 coding of Desktop apps working on RT.
But going in the future, it's iffy to me. Once Intel gets better power efficiency and connected standby on their iCore chips as well as AMD with their APUs, Windows RT with the ARM processor doesn't sound too appealing. Even with a current Intel Atom, you can get real good battery life, I think even all day battery life almost as well. But again, when a more efficient crop of x86 based 64 bit capable chips get released, RT doesn't sound real good. It might just be a stopgap measure until then though I guess...