At this point I've owned most of their newer offerings except for the CS-450 and 600P.
Echo made some real "turds" over the years, and some that are just OK. The just OK models being the CS-440 and CS-670.
The CS-510/520's are very strong runners.
Any of the reed valve engine models will fall into the "turd" catagory, CS-300, 301, 3450, 341, 346, etc.
The CS-330's and 360's are piston ported, and much faster that the other top handle models.
All of the older models with the spark plug down by the handle are pretty slow. I bought a one back in the early 1980's, and quickly got rid of it in favor of a Husqvarna 480CD, which I still own today.
It appears that Echo is hammering away at the current market with some better saws, at least in terms of power and cutting speed.
They still offer some "low end" stuff, but even those are great little saws. The CS-370's/400's are nice saws. I use them for limbing, light, great torque and very fuel efficient. They are NOT high speed designs, but pound for pound and dollar for dollar there isn't anything out there any better in that price range, IMHO.
Just for comparison purposes, I've owned two CS-670's (dumped them on Ebay), and still have a completely rebuilt CS-6700. I never use it (it's actually a "loaner" saw these days), and none of them would hold a candle to my Husqvarna 262XP or 268XP.
In contrast, my CS-510 cuts equally as well as my closed port Husqvarna 55, and has been dead solid reliable now for quite a few years. I use those two saws more than any others in my line-up these days, light, very fast, and easy on fuel......Cliff