Compare it to Debian:
Stable 1.7 branch or Debians Woody - Robust, but old, releases are now few unless major problems are found.
Testing 1.8 branch or Debians Sarge - Good, fairly stable, but just not ready yet to take the spot of 1.7, borrows things from 1.9, and 1.9 borrows things from 1.8
Development 1.9 branch or Debians SID - If you install this on a production site, your either insane or Dennis G Daniels Be prepared to do some debugging, and fixing up.
My recommendation: 1.8 which has now become my own preferred choice and runs all my tiki sites except .parkersworld.org ones (Thats a multi-tiki based on 1.9)