I tried upgrading both our appliances to HF6 and they both failed and said to contact support...not pleased
I was told it was basically a whole new version of the operating system (64-bit instead of 32-bit), so it is not a typical "hotfix."
I would recommend having a backup of the configuration available before you proceed.
My experience was that the configuration was wiped-out, but that may have been unique to our situation.
We went through the upgrade with no issues, it took approximatley 2 hours and we did not lose the configuration. Make sure to backup the configuration and apply the pre-upgrade patch.
It is not at all uncommon to see installation times of over an hour. The fastest I have seen is about 20 minutes, but that was a system was being set up brand new. On the other hand, I have seen one system take over three hours.