I use several of them at home (Devolo). The main concern I had with them is the signal leaking out on to the local grid or worse it leaking in and reducing my bandwidth! but from what I've read the signal is stopped by your meter so won't go beyond that, and I've never seen any other signals leaking in.
Localy, yes it should all be nicely encrypted as long as you've taken the time to set it up properly (this job took a few mins for me). And all though it would be possible in theory to sniff the power to grab the traffic and maybe decrypt it, if someone can do this they're in your house and you might have bigger issues.
As long as your wiring is good you should get a decent rate. Home Plug AV standard is 200Mb, I routinely get 180Mb, but this is shared with all devices (think of you house as one big hub)
CCNA, 100m Swimming cert.