Since it's free, I'd at least get the upgrade code, in case you change your mind. You just need to redeem the code. You don't have to install Mountain Lion or even download it right away. You have only until the later of August 24th or 30 days after you bought your MacBook Air to cash in the upgrade.Lion works great on my 2012 MBA.
I keep hearing everyone keep yapping that we all should run Mountain Lion.
I looked at ML's list of new features and almost all of them appear to deal with iCloud and better integration with iDevices.
Since I don't use iCloud, and have no iDevices, the upgrade would not yield me any results.
IMO, if Lion works and you have no need for iDevice integration, ML seems un-needed.