What does 1.09 fix? Shouldn't there be a sticky telling us this? If it's not essential, I'll stay at 1.08 to avoid bricking mine.
First - if you want to know what was fixed or added in a firmware release - or for that matter anything else pertaining to that firmware release, the place to check is the firmware release notes. You will find those on the firmware download page.
Second - 1.09 fixes nothing, it adds online backup.
Third - there is no inherent danger in upgrading to 1.09 - or maybe I should phrase - no
more danger than any other firmware upgrade. If you are using any add ons that modify the standard firmware, manually edited your config files to change the behaviour of the standard DNS-323 - then it might be a good idea to return your DNS-323 to a standard configuration before doing the upgrade.