For "ultra low" power, TI's MSP430 really is amazing. Many years ago, I used them in a couple projects, coded completely in MSP430 assembly. To really leverage their best low-power performance, you need to design your software in unusual ways to tightly couple with the MSP430's features.
I tried Energia recently with one of those Stellaris Launchpad boards TI sold initially very cheap. That chip is really incredible (but not at all low power), or at least it seems that way from the specs. But software-wise, Energia barely supports it. You can't even use the USB port, only hardware serial. Nearly all Arduino libraries are incompatible. I got the impression Energia might support some of the MSP430s a bit better, but everywhere I looked on their forum people were discussing baseline Arduino functionality that doesn't work.
The reality is most microcontrollers are really only well supported by Keil and IAR's (very expensive) software. Historically, the old 8051, AVR and PIC are really the exceptions. Now with ARM Cortex-M and boards like Teensy3 and Arduino Due, the situation is improving for some ARM-based chips. Maybe long-term efforts like Energia and Leaflabs Maple will eventually bring good and easy-to-use support to most ARM chips. But if you look at those other environments, it's pretty clear that maintaining a fork of Arduino on non-AVR hardware is a lot harder than it looks. It's pretty easy to get the essential functions like pinMode() and digitalWrite() working, but keeping up with changes in Arduino's API and maintaining compatibility with the ever-changing universe of 3rd party Arduino libraries is much, much more work.
