I am still using FCP 6 and it works great on my 2013 11" MBA with i7 8gb/512gb. Also worked fine on my 2011 13" MBA with i5 4gb/256gb. Rendering is faster on the new machine, and the fans kick into high about 5 minutes into a long render.
I would love to upgrade to FCP X, but FCP 6 is currently meeting my (rather modest) needs. If I get involved with any big projects, I will probably upgrade - but that might also make me think about getting a bigger/faster machine too.
But I love being able to work with video on this little machine, and it's about 3x faster than my old 2008 15" MBP. I am shooting full HD with Sony XDCAM EX.