Not bad but not perfect!

Works great - but not everywhere! The caveat is that these adapters CANNOT be used with powerstrips (that is shown in the set-up diagrams) or with wiring that acts like a powerstrip (not stated in the book)! I have two offices that use modular wiring (as in a lab) where one can plug a number of items across a long work area. These adaptors apparently "think" of that wiring as a powerstrip set-up and I cannot get the Ethernet signal to move across the wiring. When used in a more standard setting, as in an office with wall mounted duplex receptacles, no problem.
I do have a set up with a Mac G-5 feeding an Ethernet signal to an AirPortExpress downstairs via the house wiring - it works fine!
So there are circumstances where these adapters won't do the job. Check your wiring layout first!