GA means General Availability.. meaning FINAL stable version.. this is the real deal... the goods. Non-beta. Yes you can use ASL6 and 5.2 together. I have been using ASL 4 to ASL 5 and ASL 6 beta no problem.
Yep, v6.0 is awesome! I've currently got 6.0 on one side of an IPSEC Tunnel & v5.204 on the other side and everything is working great. I imported my backup from 5.204 to the fresh install of 6.0 and everything, including the tunnels, came right up.
SE joke or extended public beta, that is 95% stable with only minor bugs. Well, like 99% of the software available on the market. /SE joke
Experience tells, that you should never ever use something with X.0 version number, in any productive eviroment. Only switch if you urgently need one of the new features, otherwise wait a few more months.
