Looks as if the Arca Noae main server has not been updated (or: upgraded?) to handle the expected traffic 
. Apparently that is not included in the "steps to upgrade our infrastructure", although everybody must log in on that server in order to be able to click on the upgrade button.
It was, but not enough to handle the unexpected number of users.
Is there anything in it that is a 'must have' option?
That depends on what you mean by 'must have'. If you are happy with what you have, probably not (and I understand that there will be a 5.0.9 release). The biggies are:
- Better support foe DOS/WinOS2.
- Support to run on modern (EFI) hardware. (required for the DOS/WinOS2 support).
There are more things, but most are included in the 5.0.x updates. If you don't need those things, installing 5.1 will not do much different than installing 5.0.8 will do.
