PreUpgrade works great when your upgrading from a relatively new Fedora install. Systems which started on Fedora prior FC5 made a very small /boot partition by default (only 30MB). This poor planning by the Fedora team pretty much prevents you from using PreUpgrade. For these systems Anaconda or Yum is the only way, unless you re-size your /boot partition.
Many would argue that you could just stick to Yum (for remote upgrades) or Anaconda (for local upgrades). For local upgrades, this is a perfectly understandable argument. However, for those of us that like doing remote upgrades (or have no choice but to use them) Anaconda just isn't an option. We could continue to use the unsupported Yum option, but it would be nice to have something that doesn't require so much tinkering.
Eventually, resizing your /boot partition will become a need, might as well plan for it now and get it over with.
Resizing your /boot partition is somewhat easy and straight forward, but it will take a very ling time to complete. It involves booting of of a live CD, running GParted, shrinking your LVM partition, moving it up, and expanding your /boot partition. It's the "moving up" part that will take forever to tun because it literally has to move your data up a certain amount of bytes.
For those remote users that have no access to the system, you have no choice but to have someone load a Live CD, boot off of it, and turn on remote desktop. It sounds painful, but it will be worth while.
Once your /boot partition is big enough (I'd probably make it 200MB) you'll be able to run PreUpgrade.