Just got the 11" Air delivered today, and it's pretty amazing. As above, my 2008 2.5 MBP ran Logic pretty well for what I need (a good number of big virtual instruments and plugins, and a reasonable number of audio/MIDI tracks, but nothing extreme), and I don't think I even pushed it to its limits, so the new Air with 4GB RAM and the 1.6GHz Core i5 should be absolutely fine.
The machine is actually bigger than I was expecting (hadn't seen one in person before ordering), not quite as tiny as I imagined; but it's definitely extremely portable and very light. Very manageable to use around the house, throw in a bag, and take gigging.
I don't find the screen small at all - it's relatively high resolution, and fits a lot on it. It feels very natural, not uncomfortable at all, and I got used to the higher res so quickly that when I went back to my 23" cinema display the text etc. seemed unnecessarily huge!
The SSD is just fantastic, very quick and responsive; the machine basically screams in everything I've tried so far. I'll give Logic and MainStage a go tomorrow, but I'm optimistic! The only negative I've seen so far is the battery life, which isn't great (think I got about 3 hours from a full charge today, with 40% brightness, keys backlight off and doing only light things), but I was expecting that; also the graphics/animations in the OS are a bit choppy in places and not as fluid as they could be, but it's not a problem.
Overall I absolute love the machine and its size, and I think it should be great for relatively light use of Logic and MainStage away from home.