I use the Surface RT a lot, but my wife uses it a lot more and loves it. She mostly uses it for browsing the web, but I use a lot of baby-oriented apps - flashcards and music games, FreshPaint as well, and I'll tell you, I can't believe how quickly my now 6-month old daughter is learning to use touch-based devices. She loves the Surface and my Nokia Lumia 920. She doesn't know what to do, but she knows she touches stuff and stuff happens. She smiled and laughed the first time she managed to make a FreshPaint stroke with her hand without just moving the canvas. I wish there was a mode to lock the canvas so that it took just input from the hand though, then she would be able to finger paint more easily. Right now it's hard because she usually just tries to slap both hands down and that enters drag or zoom mode, more often than not.
I also show her the Travel app sometimes, and she loves that as well. Looking at the pyramids through the Surface, like a window - AWESOME. In case you haven't seen or tried it, it moves the Photosynth panorama around as you hold and move the Surface around. Incredibly fun.
All the Twitter apps suck now, because of the limits Twitter imposed. They never seem to work anymore.
I use a mix of other applications, like Stanford University's app and lots of reference applications like Study Online, StumbleUpon, various cookbook apps - I like AllRecipes app with video.
Anyways, there's a lot out there that I haven't tried. The reason being is that they all run full screen and taking up a full monitor is not acceptable to me 90% of the time. So they don't get used at work. It's just not acceptable, sorry MS. I need to be able to scale down the windows to the size I desire.
So, speaking of that, it's time to try StarDocks ModernMix application and see how it fares.
Also, had to say, a lot of these programs are really, really buggy. There's usually no refresh or anything so sometimes things just don't do anything at all, and you have to restart them. There's a lack of status from those apps to tell you what they are doing (usually getting data from somewhere, and it's just not coming through).
The Mail app, I use on the Surface a lot, and on the desktop sometimes. However, it's really buggy on the desktop because somehow the connection to an Exchange account totally messes up the contact info resolution.