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 href="http://www.threeten.org/threetenbp/">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?