I've been using the Luminary Micro Cortex-M3 core for awhile now. It's pretty cool, and has some nice power to it for when you're running more advanced stacks. I also started developing on TI's new Cortex-M4F Stellaris processors. The benefits of the new processors:
* Faster - standard up to 80MHz (though single-cycle flash only up to 40MHz)
* Floating Point processing
* Same or lower cost than the comparable Cortex-M3 processor
* Lower power - uses 65nm process instead of the 250nm process used on the M3 line
The best part of Stellaris (both old and new) isn't the chip, it's the software. StellarisWare is a hardware abstraction library for these processors and makes firmware development MUCH faster. It is pretty comprehensive (very few things you can't do in stellarisWare) and reasonably well documented.
The downsides of the Cortex-M4F:
* No Ethernet support
* Weird low power mode - "hibernation" but it doesn't keep the RAM alive, so you have to save your application variables
* Higher power consumption than MSP430.