Developer here: Viscosity 1.3.3 works fine under the current Mac OS 10.7 GM release (and hence presumably the full release when it becomes available).
The only known issue is that the tun/tap drivers (not Viscosity itself) may cause a kernel panic when waking from sleep if the network environment has changed. We've already worked around the issue, however as it is such a rare occurrence most users won't see it occur. We'll be pushing out a version update with a workaround when 10.7 becomes available.
As for the notes added by an anonymous user regarding "big changes in Lion firewall", they can be safely ignored. As a VPN client Viscosity has nothing to do with the Mac OS X firewall, and in addition we have had no trouble applying traffic rules for VPN traffic (for users into that kind of thing).