Have shifted to running Logic in 64bit mode for a few weeks now, and I'd like to report that.... nothing has changed.... :P Only one AU plugin is still in 32bit, AAS's Strum Electric Guitar, won't be updated soon, and I don't use it much, and appears to work through the bridge. Finale's playback plugin appears in the 32bit bridge but who uses that in Logic? :P
The last package I had to shift to 64bit was XLN's Addictive Drums. Erm, nothing to say there. They've got a nifty new installer that downloads patches and all from "the cloud". Everything else worked out of the box, and my lastest saxophone track ran AD without any hickups.
Can't see any advantages either, I don't use that much ram. I had 7 instances of VI Pro running, AD as well as one instance of Kontakt (for the triangle) no ram issues. Yet.
The only other virtual instruments I use - that are already 64bit out of the box - are VIs running with the Aria sampler. So, all of my Garritan (sorely underused nowadays now that I have Kontakt) and Plogue VIs ran with nary a hitch.