Do a search, there are several good threads on those saws. I own a CS-590 and 600P. Got them WAY under retail on Ebay, new in the box.
Both are very strong runners, and all stock aside from removing the limiter caps to give them a little fuel and I ground out the inner portion of the deflector/guide on the muffler as it's just a tad restrictive. The muffler itself is very well designed and I really see no need to modify it in any way.
I also did some time cutting/testing between my Husqvarna 262XP and the Echo CS-600P. The 600P was just a tad faster, the details are in a thread I put up here about a month ago.
Even though the 600P runs with the 262XP, I'm not doing back-flips or anything of the sort. The saw is brand new, and we're going to beat on it pretty hard over the next few years to see how it holds up? Not expecting any issues, it is a very well made unit.
As far as differences in the CS-590, 600P, and 620P, they all share the same size engine, but different pistons and cylinders were used, a quick look at the IPL's will show different part numbers for each model and again different in certain serial number ranges. The early 600P uses a different P/C than the CS-590, and 620's are again different from any of the CS-590's and 600's. They also list quite a few different coils for them, depending on serial number range. I suspect the CS-590's are "limited", simply because that coil is quite a bit more expensive than the 600's/620's. If it is limited, it's not low enough to be "annoying" like my CS-360T. With the carb set correctly I can't really tell if the CS-590 is "four-stroking" or coming up on the limiter, so good to go there and no need to search out an unlimited coil for one.
The best thing about these saws is bang for the buck, you are really getting a LOT of saw if you can snag a CS-590 for about $350 or a tad less shipped to your door.....IMHO.......Cliff