I've been using 64bit Calibre for several days now, and so far have had no problems on Win 7 64bit. I'm absolutely delighted with it, in fact, as it has finally allowed me to convert a mobi version of the Complete Works of Dickens to epub. On the 32bit version, on numerous conversion attempts it got to 47% and stayed there - the last time for 3 days! On the 64bit version it took precisely 260 minutes, although I did get a bit worred at the 47% mark as it seemed to stick there for a while.
I am curious about one thing, though. I converted an AZW3 file and it took 7.25 minutes using the 64bit, but only 6.10 using 32bit for the same file. On mobi files it is definitely a lot faster using 64bit. I will have to test out some more AZW3 files to see if it was a one off anomaly.
One extra I would like to see is a slight variation on the desktop icon so that I can tell both versions apart more easily. I've solved it on a temporary basis by finding another icon elsewhere for the 32bit for now as I intend to run both in tandem for a bit longer.
Great work from Kovid and the team.