java - Comparing ThreeTen backport to JSR-310

For some reasons, we can't use java 8 right now - we're still stuck at java 7.
However, I'd like to use the new JSR-310 date/time APIs right now, using the official backport ThreeTen.

Its homepage states:

The backport is NOT an implementation of JSR-310, as that would require 
jumping through lots of unnecessary hoops. Instead, this is a simple backport
intended to allow users to quickly use the JSR-310 API on Java SE 6 and 7.

Questions:

1. What are your experience with ThreeTen? Are there some drawbacks?




2. How compatible is this project compared to the official JSR-310 implementations?




3. How much effort is required to migrate from ThreeTen to an official java 8 implementation?




4. Is it worth to migrate now to ThreeTen and later to JSR-310 - or should someone wait until java 8 can be used, hencing skipping ThreeTen?