If your 5.5 box is working fine, I personally would wait and set up a test box with the final version of 6 (when released). If you use any 3rd party add ons for SME, you want to see if the rpms for 5.5 will work on version 6.0....My experience has lead me to believe that many addons will not function correctly on a new release of SME. If you depend on addons for additional functionality, you will end up waiting for addons to be upgraded as well. I had to wait about 5 months before 5.6 had all the updated addons available that I used on 5.1.2. So, just keep the 5.5 box running until your test 6.0 box has proven it can do all the tasks your 5.5 box is doing. In my opinion, 5.5 and 5.6 final releases had a lot if bugs/issues. Keep an eye on this forum and you will know the problems of any new release. Even if a new version is not showing problems on the forum, I wait 3 to 4 months to upgrade.
I am running several 5.6 boxes and still have a 5.1.2 box up because kernel 2.4 in 5.6 will not allow multiple ipsec (nortel extranet) pass through connections. I will keep 5.1.2 running until a kernel mod or new kernel is available that allows multiple ipsec vpn pass through. By the way, kernel 2.4 will allow a single ipsec vpn passthrough connection. If I have to run 5.1.2 for years, so be it....I have this server in private server/gateway which should be more secure as there are no open ports and it ignores everthing from the internet.
Just my 2 cents.
ryan