there is a guide on the garritan site on how to use GPO in standalone mode with logic 5.5, and it mentions briefly at the beginning that using GPO as a VST pluging in logic 5.5 is a bad idea.
However, I currently started using GPO as a VST plugin in Logic 5.5 and haven't noticed any real problems--however I'm not familiar with GPO's advanced features yet and I'm really just using it as an inflated GM synth, so maybe i'm not noticing the shortcomings yet: What are the disadvantage to using GPO as a plugin in logic 5.5??
I'm on Logic 5.5 and the bugs are annoying, but not impossible to work with. Did you do the update to the Kontact 2 Player right away? If so, then you may not notice so much, as the bugs - which still exist - seem to pop up less than the older player, and are easier to deal with.
The bugs I have come across are:
Mod wheel reset to zero: This, as you know, controls the volume on many patches. If you stop the sequence and restart, or restart from the beginning, the mod wheel data resets to zero causing the volume to drop to zero! This isn't a problem if you have mod wheel data in your track.
Stuck notes: If you stop a sequence while a sustaining patch is playing (i.e. strings), and then restart the sequence from the beginning (or elswhere) the notes that were playing stay on. I have found that this happens less frequently with the KP2 player, and even when it does, there is a "restart engine" option readily available in the new player that wasn't in the first, so it's easier to deal with now.