I am a professional photographer, and recently purchased the Macbook Air 13" base model as an addition to my desktop workstation. It replaced a 2010 13" Macbook Pro as the machine I'll use when away from the office. After a few days of testing, here are my thoughts:
Processor: The one word response would be "sufficient." Overall, the beauty of this laptop is that it gets out to the way and simply lets you do what you need to do. The processor isn't the fastest on the market, but I have never been left waiting for any processing tasks to date. As an email access point, word processor, and internet browser there is more than enough processing power to have a boatload of applications and multiple tabs open without a stutter. Web pages scroll nicely, new programs open in a second or two and when it comes to productivity tasks this is night and day better than my iPad since I can actually use the keyboard, Microsoft Office etc. The processor doesn't need to be all that powerful in large part due to the flash memory storage, upgraded in this generation to the faster 6 Gb/s connection so that read/write processes don't drag the computer down at all. While I use my desktop for all of my heavy duty processing of images and HD video, I will occasionally need to process an image or two on the road. So far I have found that Photoshop CS5 runs very nicely on this model, with nearly instant effects utilizing simple layer/curves adjustments. Processor intensive tasks such a gaussian blur related filters have a slight lag, taking approximately 30% longer than they do on my i7 920 desktop processor that is a few years old. It all gets done though, is far faster than my previous laptop, and nothing I regularly do has taken more than 2-3 seconds of processing time for an image, totally acceptable. It's the 30-40 second drags from my prior laptop that made me ready to throw it out the window. However, the few seconds of slowing is enough that I wouldn't want to process thousands of photos on this laptop, but for the casual edit it is more than sufficient. Lightroom runs smoothly as well, in large part due to the cache being on SSD storage, so even my 5D MkIII raw files at 30MB each are rendered to a smaller jpeg preview that the lighter processor can handle and render quickly off the SSD. Imports/export rendering take longer on this machine to be sure, but by that time I'm off doing other errands so I don't mind this lag. Video editing is functional as well and would likely work for most casual users, but massive projects simply wouldn't be possible on this machine for a multitude of reasons ranging from storage space to video card, screen size, processor etc. Overall, it does handle all casual productivity tasks that I need faster than any laptop I've owned, and can handle more processor intensive image applications acceptably for a laptop, though not fast enough that I'd consider using this as a full time professional machine.
Screen: The 13 inch model seems to be the perfect blend of portability and functionality for me. This particular screen is vibrant, and has the capability to professionally render the images that I need with a few caveats. Firstly, the factory set color profile for the screen looked good enough for basic use, but to my sensitive eyes had a bluish hue to it. Apple laptops are notoriously inaccurate for color rendition (increased contrast, saturation in reds and blues) so I would highly recommend that users who do much imaging work create a custom color profile their monitor. Even casual users are likely to be disappointed when actually printing images that look great on this screen (i.e. pumped up colors and contrast with a flat and lifeless print in real life). Also, for all of us that send files over the internet to Facebook/email etc, I would think it would be nice to know what your photos would look like "on average" to most of the users out there without a similar Mac screen. The "profile" utility built into OS X is worthless, and I'd recommend a software solution such as ColorEyes Display Pro, combined with a sensor such as the Spyder 4. I wish I could have assessed the default profile to find out how much it was off at baseline, but my software can only analyze custom profiles that it creates. Needless to say, things look a lot different after the profiling, and at 75% brightness I was able to get the monitor down to an average dE of 0.51 with a max dE of 2.99 in the greys (less than 1 is great accuracy, less than 5 is sort of standard to start professional work for me). I also notice that the edges of the screen on all sides have a very slight dark tinge/wave that extends for about half an inch. It is only faintly noticeable, shows up most against white backgrounds, and decreases with increased screen brightness. This isn't a defect unique to my unit, as it seems to affect all of the models at the Apple store that I went to see, as well as a few friends with Airs from years past. It would be nice if this screen were perfect, but it works and the minor screen variation doesn't effect the center working space of the image...so I think of it sort of like a built in vignette. This is a well documented complaint you can find on many forums with a quick internet search. If it's persisted into the 3rd generation of modern Air I don't see it going away soon. The resolution is also acceptable and a noted improvement over the 13" standard Macbook pro. Not retina, but the graphics on this model couldn't handle that resolution anyways. Retina display on the Macbook air will be a welcome addition in the years to come when the technology to scale everything down at a reasonable cost becomes feasible. Till then, this does the trick.
Battery life: one of my most important considerations for a laptop. This computer consistently achieves 7hrs of battery life as advertised. Screen brightness can be set to a custom level for increased battery life but I have yet to beat 7.5hrs. Surprisingly, Wifi intensive tasks such as downloading large files seem to have the biggest impact on battery life. I'd love to have a 10hr laptop at some point, but this model can get me through a busy work day every time so long as I start with a fully charged battery before.
Value/Upgrades: I am happy with the 128GB base model with 4GB ram, and made that decision mainly based on the value offered by the Apple upgrades. First off, with the base processor, all of my tasks seem to fly along with the exception of major processing such as photo or video exporting after edits. The i7 2.0 GHz upgrade would only be of marginal help, with the advantage of the "turbo boost" performance at 3.2 GHz versus 2.8 or a 11-14% increase for $100 (but requiring the $300 memory upgrade as well). I seriously considered the 8GB of ram as well, as the $100 upgrade would help to future proof the computer's performance. The problem with this, or any other upgrade over the stock base model is that you need to purchase the custom Air models directly from Apple, without the Amazon discount, with the added sales tax and recycling fee. That's an additional $150 or so in immediate outlay for privilege of even beginning to customize the computer at an additional price (or about $250 extra for 8GB of ram rather than the initially apparent $100 which would have been worth it). I have found that most Mac laptops depreciate at about 10-15% per year from the base price (any additional cost to tax etc is thrown out immediately), so selling this computer in a year or two to stay current with technology is by far the better value for me.
Regarding space, I have all of my programs (Lightroom, Office, Photoshop and a few others) on the 128GB drive with 95GB to spare. That's a lot of extra space for working files so long as they aren't media based. For that, you would need an external drive either way, as a day of photography can produce 60-70GB, and video can get into the hundreds of GB quickly, nothing that the minor 128GB upgrade would cover. The USB3 connection and a 7200RPM external drive does a great job, and ensuring that Photoshop etc are using the onboard flash storage as a scratch disk makes for great performance without compromise. Also, for storage in a pinch, the SD card slot provides an opportunity for a cheap upgrade. A 64GB SD card just barely sticks out of the side and can be had for $50 or so, while a 128GB SD card costs ~$150 with prices dropping fast. These won't have anywhere near the same read write speeds as built in memory, but would be more than sufficient for music, document, image storage etc when needed.
Overall, I'm thrilled with this machine and would recommend it without hesitation. I thought briefly about the new Retina Macbook Pro, since I could find many uses for the extra resolution and processing power. However, I ultimately decided to use this machine for its exceptional value and extra portability. The rMBP is a beautiful machine, but even with its performance it isn't close to a modern desktop machine of similar pricing equipped an i7 3770 processor, SSD boot and scratch drive, GTX 670 or similar, 16GB ram and a nice 24" dual monitor setup all for the same 2k price tag. So I'll take this with me on the road or the coffee shop, and save the serious work for my desktop with multiple monitors etc. No computer can be everything for everybody, but this Air is surprisingly versatile and ranges between best in class and acceptable for everything I've thrown at it. I'm a big fan.