I've only just started using FCPX on my MBA (last years' top end model) but so far it seems fine. I use proxy file editing and it works fine. The only difficulty I've run into is the disc size. If you're going to do a lot of HD work then 256Gb needs to be carefully managed. I've just videoed 2 days of a music festival and come back with 200Gb+ of raw video data which is going to expand once imported! So now I'm looking at external disc options, probably thunderbolt.
With the new MacBook Air you may be able to utilise the USB3 ports and certainly get the 512Gb SSD.
I also have last year's top model. As far as running FCPX goes, it's fine, although when transcoding it tends to get a bit hot. Two tricks to making it work well are (1) turn off rendering, and (2) select "better performace" over "higher quality" for playback. The screen is really too small for any long bouts of editing IMO, but this is all I have, so I suck it up.
I outgrew the 256 GB storage rather quickly, so I have invested in a dual 2TB (RAID1) thunderbolt HD recently which seems to be working well so far.
Hope this helps.