I'm going to give my honest opinion of the MBA having owned one for a month now and taken one business (academic) trip with it. I have a fear, though, that a moderator will delete my post because he/she believes Mac discussions readers can't handle earnestness, irony, or facts. That said:
The MBA is an excellent machine. While it is sturdy I highly recommend at least a lightweight case. I would personally never put a laptop of any sort loose in a carrying device. I use the incase sleeve that goes for 30 (US$) at Apple retail stores.
The machine can handle medium-duty processing up to and including light digital signal processing. So, Photoshop is not really an issue.
When you first purchase the MBA, the main problem for users who are already set up on a different machine is migrating their set ups. I did mine wirelessly using Migration Assistant. The original account was on a wired dual 2.5 GHz G5 and the process took about 13 hours. Even then, Migration Assistant converted a few of the apps into binaries that the OS doesn't know what to do with, so I had to install those softwares from scratch/download. Not really a problem or even an annoyance, but if I discovered software I needed on the road, potentially an issue. In this area, I would recommend purchasing a 30 (US$) USB-to-Ethernet adaptor to wire the MBA before migrating.
The HD is smallish. I use an iPod as a bring-it-with HD solution, but my set up is rare. If you have large iPhoto, Mail (POP) repositories, the MBA will be a bit claustrophobic.
The keyboard is a dream.
The wireless has issues. It intermittently disconnects and for no reason I have been able to discern even after trying different things for one month now. Sometimes this intermittency is very annoying (several times an hour). Most times its once or twice a day. It rarely goes more than three days without disconnecting itself.
Under 10.5.2 (not asserting causal relation, only correlation), when my MBA was using 15% of its processor capacity the enclosure temperatures would get high enough to cause the fan to rev to max. The sound is a high-pitched whine that is fine when I'm by myself though it does accelerate my fatigue when I'm writing. Others on this board have said that in public places such as library the sound is embarrassing, and I don't doubt these reports.
Since updating, I
believe my fan may have relaxed. Not sure what's going on, but the fan seems less aggressive.
The sound on the MBA has issues. For one, the mono speaker on the right side of the keyboard is awful. Using headphones is acceptable, but there is an issue with
digital gurgling when the sound volume rapidly decreases and, according to one report, increases.
The lack of a firewire port makes upgrading and backup tricky. I use USB-to-Ethernet to clone the MBA hard drive before upgrading (so far once), and the process is not quick. I have not had occasion yet to restore from the backup (so I don't know it will work), but at least one other user has
documented restoring the MBA from a Time Capsule backup. The process looks tricky and prone to it-must-be-just-so-ness. I'm crossing fingers I will never have to consult that post on a need-to-do basis. Firewire is a serious missing feature on the MBA and I hope the designers are trying to shoehorn it in now.
Finally, the weight of the MBA is wonderful. I cannot say enough about how useful the MBA has become around the house and while traveling. A machine that I can support with one hand without full exertion makes the machine mobile, not just portable. In fact, I love this aspect of the MBA, I wish the MBA were even a quarter-pound lighter.
All of this taken together means that I believe the MBA could serve as a main machine only for someone with medium to light computing needs. A serious computer user would not be able to use the MBA even with a Super Drive (which I think is unnecessary if you have networked machines with optical drives in them). You need a case and you should have a USB-to-Ethernet adaptor.
I give the MBA a B+. It is a very good machine. It has a few problems but no deal-breakers. Given the MBA features and shortcomings, I think a ten to fifteen percent reduction in cost would make the machine a much better value. At present, the MBA definitely has that annoying early-adopter tax.
As you go to market, keep in mind WWDC being 9 June. Happy shopping and I hope you follow-up with a report of what you decide to do.