To me it's worth it, but it mainly depends on how you use Finale (and other DAWs) and if you need/want any of the new sounds.

The big advantages are:

1.  New Pianos.  Finale v25 gives you a taste of the new Concert Grand Model D, and upgrading to GPO5 adds an intimate version of the Model D, as well as couple of B models.  If you keep GPO4 installed, you'll still have access to the old Steinway Grand and Duo pianos as well.

2.  Organs and Harps.  You get some extra Organ and Harp instruments who's most notable improvements are special consoles where you can build your own instruments.  I.E.  Assign Organ Ranks to various automatized controls.  I.E.  Design your own Harp pedaling schemes.

3.  Choirs:  GPO5 gives you several more choir options, including Boy and Children ensembles.

4.  Extra SAM Brass  Instruments:  There are a few nice editions to the Brass instruments.  They aren't much different from the SAM Brass in GPO4, but you do get a few extra layers to work with.

5.  The new Orchestral Strings:  These are kind of hit and miss depending on your score, and how much tweaking you're willing to do in order to get a quality performance.  The 'out of the box' setting for these are pretty bad in my opinion, but given some tweaks, they can offer some nice touches.  On the plus side, you do get dozens of new instruments and articulations to work with.  There are some interesting layered up 'group' patches as well.  TO ME, it's worth it to have these new strings, but they do require a good bit of work on my part to master them (including tweaking the sfz opcodes more to my liking and situational needs).

6.  Percussion isn't much different from GPO4, but there are some minor changes (and a bug or two as well, that you can fix if you learn a little about sfz opcodes).  Some of the instruments that used to have an un-metered key-switch for rolls/trills, have changed to a triggering type scheme.  Some of the rules for damping and velocity vs CC1 dynamics are slightly different.

Having said all that........for me it's worth it.  I use a tracking DAW as much or more than I use Finale or Sibelius, so the extras are really nice to have when doing mock-ups.  I'd say I've gotten my $50 worth for the upgrade.

I would keep GPO4 installed for a while even if you upgrade.  There might be cases where you want to roll back and use GPO4 instruments instead (Most of GPO4 is duplicated in GPO5 for compatibility, but there are a few things that aren't included in GPO5).  I.E.  If you have Sibelius, there's no official soundset for GPO5 (though I can give you my personal version).  I.E.  If you want to keep the old Steinway Pianos.  I.E.  If you want the old Timpani sound and playing style.

As for conflicts or problems with Finale...it's nothing serious.  You can still use all your other Garritan Libraries, including GPO4, mixed and matched in a project.  Sometimes people change a score over to GPO5 that was begun with some other Library and forget to refresh their human playback settings (MIDI Tool, Apply Human Playback, Clear....then reapply), and sometimes there are techniques in Human Playback that can conflict, but it's easy enough to sort out and fix.  If you don't like the default sounds that get loaded after installing GPO5 when opening a fresh score, or using the 'reassign instruments' option, it's not difficult to change that up with a little xml editing as well (I.E.  If you want Legacy Strings to load by default instead of the new GOS strings, then you'd just change the order of some things in the GPO5.soundmap file).