Upgraded my Mid 2011 MacBook Air, went smoothly and the changes are mostly invisible.
Like all previous Mac OS upgrades it just works. The machine was delivered with "Lion" installed, so this is the 4th main upgrade to the OS.
How many more will come before the machine needs replacing? Indeed, how long will it last?
The battery life is still good, it is fast enough for my needs, running a 1.7GHz i7 processor and with 250GB SSD "hard drive" there isn't much to wear out.
The only think that tempts me to upgrade is the Retina display on new Mac Books.
