Use nightlies if:
1 - You are running a testblog
2 - You are perfectly happy for any sort of breakage to happen
3 - You accept that help fixing any breakage might not be immediately available.
4 - You accept that the code "may or may not work at all"
Yes the code may be doing wonderful things, but unless you are content for your blog to act in mysterious ways, then please do not use them.
And if you do, do not post issues to the Troubleshooting forum - all issues must be posted into the 'Beta Discussion' forum.
Backing up a current installation BEFORE using a nightly is strongly recommended ..... you could lose data otherwise.