I have a MBA which I bought in September 2010 and which still runs perfectly well, although I am giving thought to an upgrade, probably later this year. In the time since I bought it (and subsequently sold my 2008 MBP), it has been my main computer and has travelled a lot with me. I can say it is easily the best computer I have ever had. Thus far, the keyboard and mag-safe needed replacing (which Applecare took care of). Other than that, I have had no problem at all (whereas the HDD failed on my old MBP, and had to be replaced under Applecare).
Re the MBA itself, the SSD drive is a lot more stable (and faster) than the more traditional HDD. Indeed, with falling prices, and, if the MBA is to serve as a main computer rather than a back-up, or travelling second computer, it now makes sense to have a larger SSD drive than what used to be available.
Likewise, as has been advised by almost everybody else, I would advise that you buy whatever RAM you can afford, as whatever is being developed now will tend to hog more and more RAM. Certainly, for now, 4GB RAM is more than enough for your current - or stated - needs and probably will be perfectly adequate for the next few years. Nevertheless, as, doubtless will be said by others, 8 GB of RAM would future proof a machine to a greater extent.