The truly great thing about the Gecko family isn't the low-power ARM M0 core: Everyone is making chips with that core these days.
What's great is how the on-board peripherals are tightly integrated into the power reduction and management schemes. You can do a tremendous amount of processing using only the peripherals without any CPU involvement, waking the CPU only for things like DMA buffer exhaustion. The configurable switching matrix interconnecting the peripherals permits several peripherals to operate in parallel, all without CPU attention. Truly amazing.
Using various members of the EFM32 family, you can have nanowatt embedded systems that are continuously active, and can wake the CPU when needed to provide tremendous computing power.
I'm a particular fan of the tiny gecko, and have the same evaluation board at home. I can't wait to use that processor in future projects at work.
The Segger J-Link is a fully functional Segger debugger that can be used as a separate tool using the debug connector, without using the on-board EFM32. There is a Segger coprocessor on the board, hidden under the LCD.
The coin battery holder is no joke: The board can operate for prolonged periods doing substantial I/O powered only by the coin battery, so long as you carefully manage the CPU wake time and clock rates.
If you don't want to pay the full price for the board, the various Energy Micro distributors often host half-day seminars where they either give the board away for free, or for less than half-price. I highly recommend attending the seminar (which is how I got my board): Learning to program multiple interconnected peripherals to work while the CPU is off, and to wake the CPU when needed takes some careful planning,and the demos are invaluable.