lots of choices, but I'm playing with BT to see how well this serial transparent stuff works.
$10 a radio is a great price. easier to use than nrf. nrf can run much higher power (I use a LNA and PA version on my hobby RC remote control and the nrf protocol is used by lots of quadcopters these days).
I used to use tons of xbees and those are $30 per radio once you add in the breakout boards that most people need.
the cool part about this bt dongle is that you can run peer to peer on a pair of the dongles OR talk serial in a stream of bytes to a phone or any bt classic (not ble) system. I was able to connect to my win7 system and android just fine (blue-term on android, or equiv).
I tried following this guide but my firmware won't take the extended AT commands that they talk about. none are taken. I can only set the password PIN and that is the only way, right now, I can set up pairs of them as ships in the night.
https://arduino-info.wikispaces.com/BlueTooth-HC05-HC06-Modules-How-Tothanks for comments on the build
the project is a lipo battery charger and I'm using lots of breakout boards to try the concept out, at first. a digipot to control the charge current on a TP4056 charger board; arduino for control, sd-card to save log data of the charge process and a 4ch a/d board that lets me measure differential voltage for the batt and diff current (across a .1ohm series R).