I have a 55, and you can see my posts below "Husqy 55 won't start" for some issues I've had with it. That being said, the 55 is a good saw, starts easily for me, and runs great with pretty good power for its size. Mine likes to gobble chain oil unless I use pretty thick stuff in it; it doesn't have an adjustable oiler. You can find the service manual on-line, which will tell you how to run a compression test if you have a compressor, gage, and fittings.
Some things I'd look for before buying a used one:
* Do a compression test, if you can. Otherwise, do a drop test - see comments in my entry on how to do it. If you suspect a problem, pull off the muffler and look at the piston. I'm told that 55's like to run hot, which results in the outlet side of the piston getting scored. You can see this easily once you pull off the muffler. You'll see rough, verical lines on the side of the piston, and maybe a rough or ragged edge at the crown. If it looks smooth and shiny then it's OK.
* Check how smoothly the chain runs on the bar when you pull it around manually. If it seems like it's hitting "high" spots, and they appear to be about the same distance apart, then the crank shaft might be bent.
* Is the air filter covered with oil? If it is then there's probably blow back from a worn piston.
How much is a used one worth? Anything more than half of a comparable new one seems high to me, unless it looks nearly new and everything checks out perfectly. Good luck.