I don't do a lot that stresses the i7 in my 2011 Air, but I don't have a sense of how much slower a Core M is and what that'll mean day to day.
Edit: chased down some benchmarks, and it's quite a bit slower than any current i-series combo, though the turbo makes up for it some (probably at steep cost to battery life).
What a bummer, I was seriously hoping to replace my 2012 MBA, but now... apparently the Core M processor will unfortunately be a good deal slower, than the one used 3 years ago. Which means I'll probably have to go with a Windows based laptop or wait for the MBP update.
The Core M *71 isn't that far off--3DMark is notably lower, but most others are pretty close.
But what concerns me is that reviews of other systems with the chip mention that it throttles itself pretty aggressively when hot. I have no idea how much that would or wouldn't hit these benchmarks. This might be best-case performance on a cool day, for the first 30 minutes.