About a month ago I sold my 2010 MBA, 4Gb, 64GB, C2Duo to a coworker for $650 with Applecare until nov 2013, an Incase sleeve and a brand new Warpsol cover, so $550 looks pretty fair to me...
I never had any problems with the machine, the Adobe CS5 suite performed fine, Word, Excel and Powerpoint did fine too. I used to plug it to my Apple Cinema 30" when I was at home and the little thing tame that huge display, movies look great... sometimes Safari was slow but was the flash plugin fault, doing the usual memory hog. The machine never became too hot, and the fans were almost silent.
It was a pretty reliable machine, light and very snappy... I sold it because I change computers every 18 months, it was my 6th Apple laptop and was one of the best... not including of course my new i7 MBA 11.6 which is a lighting beast 🙂