It's pretty cool, though I probably wouldn't spend for it, being a cheap bastard.
If you wanted to use encryption, you could set up a VM or Pi on your local WiFi network running Asterisk, and have the calls bridged, such as Asterisk <--> Jitsi, with the person on the other end using Jitsi; this would encrypt everything except the local Wifi portion, but anything leaving your home would be encrypted.
Any time a phone number touches the "regular" phone network, it must by law be allowed to be eavesdropped on (CALEA) by the government. So anything completing over SIP that used regular phone service could be intercepted. If you had encryption from the WiPhone to a shared Asterisk, such as with a group all using the same PBX then you might be able to avoid it.