I'm using one on my late 2008 MacBook that has BT 2 built-in. Unfortunately neither Handoff or Continuity are working. However it's too soon to conclude it's the dongle's fault as plenty of users with built-in BT 4.0 are also saying these features are unreliable with current beta releases of iOS 8 and Yosemite 10.10.
If people are interested in also getting a BT 4 dongle, be sure to buy one with the Broadcom 20702 chipset. I've tried others and they were either unreliable or didn't work at all. This is the one I have, and besides not working with the above mentioned features, its "normal" BT capabilities are working as expected.
http://www.amazon.com/gp/product/B00DDH4TYA/