Just bought the latest Logitech MX Master mouse, which is quite powerful for Yosemite to natively support Mission Control and Spaces with its mouse gesture button, together with the interesting Smart Shift scrolling feature.
But also encountered the problem of pairing it with my MBP Retina on Yosemite 10.10.3. One discussion thread in the logitech forum suggested to delete all existing bluetooth devices to fix the connection. However, some hidden devices remain in the bluetooth paired list for Alt-Click menu. In this case, a complete bluetooth plist delete will finally help to pair the mouse with MBP.
Yosemite Bluetooth sometime freezes after wakeup, which then cannot be turned on/off and receive connections until reboot. If you want to manually restart it, do it by command line:
sudo kextunload -b com.apple.iokit.BroadcomBluetoothHostControllerUSBTransport
sudo kextload -b com.apple.iokit.BroadcomBluetoothHostControllerUSBTransport
Or you can use my Bluetooth Restart Alfred Workflow or similar AppleScript.
El Capitan UPDATE: Above bluetooth kext restart method will make the bluetooth driver unusable in 10.11, so restart your mac seems the only way to fix bluetooth freeze.