They are more powerful than AVRs but these ARM Cortex-M micro-controllers do not seem easy to work with. See this discussion: http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=127016
Too many vendors, too many types of ARMs, too many different peripherals, too many ways to set them.
The huge diversity of ARMs seems to be their main enemy.