Hi Adam,
We are on 6.1.2 and have no plans to upgrade soon.
We started 7 years ago on v2. All upgrades went well UNTIL 6.0.9, which gave me 2 of the worst (professional) weeks of my life.
The subsequent upgrades were fine, 6.0.11(or 6.0.12?) then 6.1.1 and 6.1.2.
6.1.2 is doing very well, rarely crashes, overall speed very good. Order writing is a little slow.
But I am unconvinced that the (albeit small) risk of a major problem with an upgrade to 6.3.3 justifies the benefits.
Gene