Upgrades have the same options as patches: you can do it yourself or pay for the eZ network solution, which includes upgrades.
As with patches, our experience has been with manual upgrades only. We aim to have each solution upgraded once a year; however, sometimes a client will choose not to upgrade their site as there is no new functionality in the latest release that they feel that they need; on the other hand, some want to upgrade or we recommend for them to upgrade as the latest release includes a feature that they can take advantage of.
When considering an upgrade, we tend to not go for the very latest release; they usually have bugs that are yet to be fixed or discovered. E.g. rather than upgrading from 3.5 to 3.6, we kept waiting until 3.6.1; this version was more stable and the inevitable bugs from a new point release had been fixed.
It's important to note that releases are cumulative. You can't pick and choose to simply jump several versions in an upgrade without considering...