First a correction:
Hyperion's OS will not run on a machine with OpenFirmware. The
AmigaOne ships with another boot utility, called uBoot, which works
very similar to Linux's LILO program.
As for OpenFirmware itself, the Pegasos's OF is primarily there to
configure the system, same way that Autoconf was supposed to work.
This means that your hardware is already set up, no ugly PnP needed or
messy device configuration utilities. The front-end shell is still
somewhat primitive, but it is more than possible that in the future
there will be some kind of graphical front-end. But that is not a
high priority for the Pegasos team, as they are at present working
hard to get the Pegasos 2 out the door by September.