I've been running Fedora Core 3 for i86_64 for three or four weeks now.
My experience is generally good, but I've had to make some compromises and I've had one nagging problem.
As far as I can tell, the latest versions of Java are not well supported for i86_64 yet. So I'm running 1.4.2, which is available from JPackage and seems to me to be well supported.
Also, the vendors of the most important browser plugins haven't yet ported them to i86_64. So my understanding (and my experience, in fact) is that if you run the 64-bit version of firefox you will not be able to install the Macromedia Flash Plugin. You can decide for yourself whether that is a bad thing.
Finally, I've experienced occasional complete freeze-ups of my system (say once every couple days or so). I don't know that this isn't a flakey motherboard (which I also migrated to at the same time) or some such.
All this said, I hope more and more people will move over to i86_64, since that will encourage more support for it. And my experience is that it is NOTICABLY faster, starting apps, running apps, etc.
Best of luck,