Here is an excerpt from the spec:
Upon communication with members of the Google Talk team, it was discovered that the emerging Jingle approach was conceptually (and even syntactically) quite similar to the signalling protocol used in the Google Talk application. Therefore, in the interest of interoperability and adoption, we decided to harmonize the two approaches. The signalling protocol specified therein is, therefore, substantially equivalent to the existing Google Talk protocol, with several adjustments based on feedback received from implementors as well as for publication within the Jabber Software Foundation's standards process.
The spec document is written in an accessible language. It was easy for me to grasp the concepts and scetch out the basic interactions between calling parties. Looks like a protocol that should be relatively easy to implement in client devices.
The authors deliberately chose to push as much complexity as possible to gateways and servers. Even more so than in SIP. In several sections in the document the text points out why certain features were implemented differently than SIP, which shows the advantage that the authors had designing something from scratch taking into account lessons learned from SIP experience.
I have been told in the past from engineers at several telco operators that SIP is somewhat heavy to implement for thin mobile devices as compared to XMPP. At the time Jingle was not standardized, so the comparison could not have been complete. Still their experience was an indication that XMPP may be a formidable competitor to SIP for VoIP signaling.
For as long as both protocols are open standards, I am only expecting good things to happen for both mainstream developers and end user.
See the full text of the Jingle Specification Document.