This past weekend I installed both, thinking, "hey, upgrades are better, right?" I did, however, find two crippling problems. The first pertains to Fusion 3. When you start it up, it is slower than molasses, and the reason is that Spotlight is told to search your virtual machines. In theory, it should do this once then be happy but reports have it that it reoccurs each time you start Fusion. The solution is nicely stated in this post.
The second is that the Cisco VPN client doesn't work anymore. This is because the current version of the client (4.9.x?...) is not 64-bit capable and it dies upon startup. The solution is to use the built-in VPN client in Snow Leopard but I've heard the connection can be flaky.
