I just returned my Air 2 for an original Air. I had used the first Air for a few months previously as well. While the 2 is thinner and lighter, you really do need to compare them side by side (and really, is this a practical all day effective issue?) then you can tell a slight difference. If you use one or the other exclusively, that difference in perception vanishes pretty quickly. The 2 is a bit snappier but for the majority of tasks such as browsing, email, photos and many, many other apps that I use personally and professionally the performance difference was minimal if any at all. Even heavier apps like Pixelmator run extremely well albeit a second or two longer to render a heavy effect, if at all. The screen is nice but it didn't blow me away in terms of making the original air look like a charcoal sketch. Both are great screens and the minimal increase in potential perceived improvement is very minor in my comparison. The original Air actually seems to have better touch response as I found myself needing to tap certain elements two or three times on the Air 2 to get it to register.
The Air 2 of course is priced well at $599 for the 64GB wifi model but wight the sale at Target for the original Air 32GB wifi (plenty storage for my needs) at $449 plus the RedCard discount bringing it down to around $425, I just couldn't see the difference in justifying nearly 1/3 more money to get essentially the same job done.
I'll be hanging onto this Air until next year when hopefully we will see another significant change.