Is Samsung good about updating the OS on their phones?
I think the most you can expect from a manufacturer is 12 months of software upgrades, 18 months at best, and they will be 3 months after Google's release. This is reason #1 why I went to the Nexus line, not to mention that its easier to develop on. So for the GSIII, Key Lime Pie for sure, whatever the L release is, 50% chance at best.
And before the Apple folks chime in on having iOS to eternity, Apple cripples the release based on hardware capabilities for the older phones. So Apple folks aren't getting full upgrades. Google's X.X.X updates support that, but not an X.X update. It's usually a full rewrite.