I wouldn't see it as an Arduino competitor as they're different beasts mostly aimed at different use cases.

The Teensy also has a DSP on board which can be useful in many applications, from music to signal processing. Here's a SDR radio built around a Teensy some years back. http://theradioboard.com/rb/viewtopic.php?f=4&t=6641 Does anyone know of any kits like that around?