I had both for a few months before I decided to sell my M11x and stick with the 11.6 MBA. They are two different computers for different purposes and the M11x is definitely a little beast. But I found it to be too heavy and thick for an ultraportable, and I soon realized I didn't have the time to game on the road as much as I thought.
When the new MBA came out, I knew the 11 was exactly what I was looking for: a super thin, lightweight computer that had decent power and could even game a little in a pinch. I debated for a few months because I just bought the M11x in August of last year, but in December I bought the MBA and I found it to be the most versatile computer I've ever owned (for my needs). It's much more powerful than my old Acer netbook, has a fantastic keyboard, excellent screen that really puts the M11x's to shame, and can actually game a bit. Honestly I've been really surprised at what a champ this little guy is at casual gaming (SC2, Amnesia and the latest PoP in Win 7).
After a few months I realized the MBA was everything I needed and that my M11x was just collecting dust, so I just sold it a few weeks ago. It was a great little computer, but it just didn't fit my needs as well as the MBA did and it got a good new home. Looking back on it, I probably wouldn't have bought the M11x if the new MBA was out last summer.
Though I do miss the lighting of the M11x, especially the keyboard!