I'm a fairly new owner of a Echo 590, but I've used it enough to know it is a good saw. I was really hoping to be in the woods working by now, but this snow is crazy. I was able to run it good back in the fall and early winter.
I'm not sure if I would buy one used or not just because they can be had for such reasonable prices. Brand new they can be had for $350, there's one on ebay right now, free shipping. I bought mine in the fall for even cheaper. So they can be had brand new in that $300 to $350 range if you look. $200 would offer a decent savings, however, I'd definitely want to look at it good. You never know though, sometimes people buy saws, use them once, and let them sit in the shed for a few years.
I'm not a chainsaw snob. You give me something that works and cuts, I'm mostly happy. The Echo 590 is definitely one of the nicest saws I've owned. There's really only one issue with them, and that is the air filter. Some people downplay the issue, and they may be correct, but the air filter design they use is A grade trash. It will suck in wood dust, and I do not consider that acceptable. There is a super cheap fix, you just add an O-ring between the filter and the carb, as well as the stud. That should fix the problem. I ended up buying a foam filter kit myself. Besides that, they don't seem to have any flaws. Their carburetor setup is fantastic, and it's a simple job to pop the limiter caps off leaving you with a fully adjustable Walbro carb that doesn't need a special screwdriver. It has an adjustable oiler with no real issues. I don't know if there is anything better for $350 new.
Used saws are a whole other ballgame. There's nothing wrong with a lot of used Stihl or Huskys. Go old enough, and you can find some really nice McCulloch and Homelite too.