Disclaimer: I purchased a Surface Pro for personal/school use.
The RT was, quite frankly, a bad idea.
The pro has a lot going for it, if you're in the market for a moderately high-powered x86 ultrabook with a stylus and touch screen. Basically, it's the cat's pajamas for people that need something exactly like that (I do audio recording and some graphic design work when I'm out and about), and it's an overpriced novelty for anyone that doesn't. No remorse here, I love the thing, but I know I'm not a typical end user and there aren't enough people like me to support the kind of R&D that goes into this sort of device.
The RT takes all of the advantages the pro has, and throws them out the window.
You're left with an underpowered, oversized tablet with an underwhelming user interface and no applications to speak of. It's pretty much the perfect storm of uselessness. Which makes it no real big surprise that it's selling badly.
At least with the pro they can sell it to the developer/designer folks (my sister, who does photoshop work on a regular basis, was drooling all over it) instead. The RT? Not so much.