I love Lion. It runs perfectly smooth, and the full screen apps and swipe features are ideal for the smaller screen.
I know some people had issues with Lion after upgrading, but since the new Airs came with Lion preinstalled it is clear it works as expected.
Plus, in the coming few updates, Lion will be updated to iron out the kinks (10.7.2 and onward), so it will only get better from now on.
If you need a laptop and you want an Air, by all means I think the 2011 versions are great.
If you want to save a few bucks and you don't need the extra processing power, then get in on a refurbished model of the 2010 one and save a couple hundred bucks.