As far as I know, they haven't released a good solution to the VPN problem yet. People have said that the posted fix will fix some connections, but break the ones that were working. I think the fix disabled PPTP masqing for the sake of incoming PPTP (though I haven't really been paying that much attention to that stuff). From what I've seen here, I'd steer clear of 5.6 if you rely on VPN.
5.5 and up use Mailfront. This changes the way some things are done. If you have any mail customizations, you may want to go to 5.5 now and work out the mail issues, then the upgrade to 5.6 when it's fixed will be smoother. I stayed at 5.0 due to some early bugs with 5.1.2, and upgraded to 5.5 when Damien released his SASL addon to support our remote mail needs. The bugs in 5.0 didn't affect us but the new ones in 5.1.2 did, so it seemed silly to upgrade.