Summary: In the past, Apple has charged $129 for upgrades with far fewer improvements than this, and that price upgraded just a single system. At $30 for all the Macs in your world, the only reason not to upgrade to Lion is because you rely on old PowerPC-based apps that won’t run on it. Otherwise, it’s a more than fair price for a great upgrade.
Excerpt: In a decade, Mac OS X evolved from a curious hybrid of the classic Mac OS and the NextStep operating system to a mainstream computer operating system used by millions. It was a decade of continual refinement, capped by the bug-fixing, internals-tweaking release of Snow Leopard in 2009.