I got my 13" MBA in July right after it came out. It has 4GB RAM and a 256GB SSD. Honestly, I've never owned a computer this incredible in my entire life. First let's talk specs, I chose to get more space instead of more RAM because I decided it would be more useful in the long run, especially for my computing tasks. I think 4GB of RAM is enough for me because the SSD adds a lot of speed. You won't see any lag unless you're doing heavy video editing, although honestly I haven't even tried that so I can't say for sure. I've never experienced any type of lag on this computer, it's insanely fast with Mavericks. On top of all of that greatness, it has the best battery I've ever had on a laptop, I get 12 hours out of it!! And of course, it's VERY light and thin and doesn't add much weight when I travel with it. I went ahead and got Applecare on ebay for $175 (not a code, an actual unopened retail box) so I'm protected until July 2016. I think I'll be keeping this thing for a while. As far as I'm concerned, this is the first REAL replacement for the 12" Powerbook G5. Although I'm sure many will disagree. 
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