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" for navigation. AirDrop wireless file sharing.
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. Otherwise, it’s a more than fair price for a great upgrade.