Conclusion: Unless you have powerful reasons for using anything else, you probably ought to use OS X.
Pros: Easier to use and more powerful than ever. Tablet-style features include automatic file-saving and resume. Multiple versions of documents saved automatically. iCloud backup, syncing, remote access. Find My Mac. Views and prints Microsoft Office documents with no extra software. Trackpad "gestures...
Cons: No support for old PowerPC-platform apps. Distractingly cute interface on calendar and address book. Launchpad duplicates existing features.
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.
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.