Since I got the Air I've been carrying it around on a regular basis, largely foregoing use of my other ultraportables.
Prior to this my recent ultra/moreportable history has been the Sony SZ / G11 / TZ / UX and Dell D430. I currently have 4 UX's (3 390's, 1 490), 2 SZ's (71VN's - Penryn 2.5Ghz in a 1.8Kg frame), 3 TZ's (180, 21, 22) as well as a remaining G11 and D430. These are spread around various places I live and work in. I had planned to do the same with the Air and bought two to start with, but rethought the situation given the specs and decided to start with one.
It's a beautiful machine, but curiously half-assed in almost every way and seriously compromised in others. I like it on a superficial level, but I can't say I'm impressed by any particular aspect of it apart from the image and the feel of the device. The build quality has gone up over the MBP (but then I was never impressed by the MBP's build), but there are clearly still a number of engineering lessons Apple has to learn from other makers.
While Leopard needs a decent processor to be snappy (especially for media use) and it was clearly the reason why Apple opted for a shrunk C2D over a ULV, I think shoehorning the 1.8 into the size was a bit of a flawed compromise. Performance is nominally very decent but it can become unstable once heat issues comes into the fray, and once a certain threshold is reached the Air is actually noisier than many ultraportables. Perhaps the 1.6 is a slightly better compromise, I'm not sure as I don't have it. Either way, similar basic Office-type app performance compared to a 1.33 C2D ULV running Vista on an SSD is not as earth-shatteringly superior in everyday usage as things might seem, especially for an ultraportable - i.e. a second travelling PC sense - the faster processor, especially with the compromises it comes with, is of debatable benefit.
The other functional compromises compared to the other more genuinely cutting-edge-engineered ultraportables from Lenovo, Sony and Toshiba et al are obvious and evident, so I'll not go into them here. The relatively short battery life of ~3.5 hours in daylight-viewable mode with wireless on wouldn't be a huge issue if charging was as quick as Dell slim&lights for example in order to allow topping up of the charge during the course of the day, but it isn't - and neither is the battery removable to compensate for that. The flexibility of the TZ's for example can't be overstated: you can opt for a 6+ (real) hour battery or a 10+ (once again, real) hour battery, with the 10+ hour TZ being barely a teaspoonful of sugar heavier than the Air.
There are also less evident compromises. A Megabit LAN interface for example, which given the uses for which a wired LAN might be put to nowadays seems somewhat redundant. And as for wireless problems, as clayj and others have related I have frequent problems connecting to and keeping connections to certain wireless networks and SMB shares that my Vista machines have zero problems with. This could be an OS X issue more than an Air issue, since I have similar problems with a limited selection of other Leopard gear - but nevertheless, such a critical part of its connectivity should work flawlessly, especially in a single-manufacturer ecosystem.
The Superdrive works fine, and I don't even care that it seems to be specific to the Air - but my problem with it stems from the image-centric nature of the Air, and that the Superdrive's design language is more in tune with the Mini than the Air. Standing the two side by side, the drive does not evoke a coherent design statement. While on any other notebook it may not be a big issue, on such a design-driven machine it screams mismatch. The USB cable of the drive is also free to whip around and scratch other items, unlike other separate drives made by better manufacturers. Basically, it has 'afterthought' written all over it.
The screen is fine, LED is what I've been used to on ultraportables since my Sony G11 of early '07. However given the screen size, I'd like better use made of it in terms of resolution for mobile use. I know Spaces can alleviate this problem, but 1440 x 900 wouldn't be a huge stretch on a screen of this size - and indeed, Lenovo offers it. To be fair to Apple though I have the same problem with the Sony SZ, although the TZ offers a slightly higher 1366 x 768 resolution in a more baggable size.
I like the look and pitch of the backlit keyboard especially with the Air's key colour combination, which is IMO a far more practical colour for backlighting than the MBP. But I don't like the 'straight down and thump' feel of the keys. I can get used to using it fine, but I'm accustomed to more refinement in the key action, even on ultraportables.
Ultimately though, as pretty as it is the Air is yet another piece of marketing brilliance from Apple for the credulous that has failed to impress me on a fundamental level. As an attention-getter, it will continue to be fished out of my bag for public engagements (until something better looking comes along) but as it stands, I have to conclude even at this stage that it's inadequate and unfocused as a working ultraportable for me, no matter how nice it feels to hold.
I suspect I'll be back more or less exclusively to the Sony TZ's (a refresh to the TZ31 1.33Ghz models will be soon) and my recently refreshed SZ's for the foreseeable future, although the Toshiba R500 and the Lenovo X300 arriving soon may be in with a chance.