I got on a kick a while back to use the RFDuino. The size was appealing and I imagined it could do some things it can't. For BLE devices, there are two roles, Central and Peripheral. Watches are peripherals, as is the RFDuino, an iPhone is a Central. So, after much head scratching, I've realized I can't connect an RFDuino directly to a watch. Kind of a show stopper. Other issues with the RFDuino, which may or may not be imagined:
1. If you use the one and only serial port for something, you can't use the USB for debugging output. Typical problem with single Uart microcontrollers, but this is why I like the Teensy and Mega ADK. It's hard hacking when you're running into constraints. In this case, you run out of options for debug output, and are down to blinking led's.
2. The programmer is cumbersome, and you need to unplug the board from the circuit to re-program.
3. When you google this product, mostly what you get are items related to the kickstarter. Lots of people bought these, then went back to their caves and vanished. There are few write ups on actual projects out there.
Another strange bird. Will make you appreciate what the Arduino empire does well. Constant "service" calls required to keep Serial output open, otherwise "stuff" just shuts down. Want a simple delay statement, forget it, you'll lose your serial out. Also, I've noticed that uploading new code, doesn't always happen, although it says it did. The compiler for this thing is slow. And then there's the lack of memory. But, we're all using this for a very good reason, so deal with it...