As usual we have been testing the latest version of Capture One on a variety of Mac machines under both 10.6 (Snow Leopard) and 10.7 (Lion). We test Phase, Leaf, Mamiya, and Canon tethering. We do extensive LCC testing and work with large sessions.
In this case we are happy to report that Capture One 6.3 has passed all of our testing with flying colors. Nothing is perfect, but we have found 6.3 to be rock solid, very fast, and otherwise very polished.
For customers with 10.6 or 10.7 we strongly recommend Capture One 6.3.
The usual caveats apply. See the bottom of the original Capture One 6.3 Release blog post for those caveats.