I have a U2 (basically the same as the U3 with less ports), and the thing is so good I used it as a desktop for a couple of weeks (http://the.taoofmac.com/space/blog/2013/02/10/1230) - running NetBeans at a fair clip, to boot. The only thing stopping me from getting a couple of U3s is lack of cash, and I heartily recommend it.

The Pi is... Sort of mediocre as a desktop (even as a thin client) and too complex for most hardware projects. We ran a number of them as digital signage clients for a while (https://github.com/sapo/digital-signage-client) and eventually moved to Android boxes (https://github.com/sapo/android-signage-client) because even with the recent improvements to WebKit and X the browser simply wasn't up to par.

Worked reasonably nicely with videos, but I keep wondering how much more useful the Pi would have been (for signage, mind you) if they had released their internal Android build.

Edit: forgot to mention I have five Model Bs in a cluster (http://github.com/rcarmo/raspi-cluster) where I run distcc and toy with Java/Clojure/Hazelcast. It's fun, but requires a fair bit of patience.

You can check the load here: http://rcarmo.sl.pt:8888