I have owned the following Echo saws:
CS-300, CS-301, CS-360, CS-440, CS-510, CS-670, CS-800
Here's the unbiased opinion(s).
The smaller CS-300/301's are reed valve engines, SERIOUSLY underpowered, very, very slow, but dead solid reliable.
The CS-330 and CS-360's were built to replace the smaller 300 series saws, they have piston ported engines, and run over 14,000rpm's. They cut at least twice as fast with good torque to go with the chain speed.
The CS-440 (stolen) was OK, but marginally capable of running an 18" bar. It was much better suited to the 16" bar that it left here with (for whoever stole it, if/when caught, I have something for you:
)
Sorry for thinking out loud.
Anyhow, the CS-510 is an EXCELLENT saw, we ran it side by side with an MS-250, outpowered it at every level. The MS-250 I used was almost new, owned by my next door neighbor, so I didn't get a chance to adj the carburetor/hand sharpen the chain, it may have faired better against the 510, but as delivered it was not on par anyplace.
I ran one of our 510's the other day against our Husqvarna 55. The 55 runs a bit faster, but doesn't "grunt" quite as well. I'm very partial to the 55, it is probably the favorite saw in our line-up, light, fast, powerful, good feel to it, effortless to start, and never complains!
The 510's we have get a LOT of use, they have excellent power to weight ratios and very, very thrifty on fuel consumption. To date, durability has been perfect, no problems in any area, aside from the fact that one of them left the factory without a fuel filter on the supply hose, and required a carburetor cleaning/filter installation. I would NOT recomend a bar longer than 18", they are very, very well suited to a 16" bar, fine with an 18", anything bigger wouldn't make it.
The 670 is overrated. It came with a 24" bar, much better suited to the 20" bar that we currently have on it. Whoever thought they could run a 24" bar with that saw never did any SERIOUS cutting with it! I can compare it against our Husqvarna 268XPS, well, sort of. The 268 will outrun it at every level, at least 20 percent more power and cuts a LOT faster. I still like the 670, it's a good basic "workhorse", runs pretty fast, decent torque, it would be a really nice saw with an 18" full chisel bar/chain set-up.
We also have a variety of other Echo equipment, string trimmer, garden tiller, leaf blower. All of it is excellent, never once failed us in any way, and they see quite a bit of use.
Overall I believe the company has responded well to the current market. They have lightened up their saws, went to piston ported engines, good quaility and durability, combined with reasonable pricing. This all has helped their company to get a good foothold in the current market. My opinion, FWIW, Echo is the best choice for reasonably priced reliable saws and other equipment. If you want professional quality stuff for commercial use, step up a few hundred dollars......Cliff