Have two of them, the 58-V 16" bar (CS500) and the new 56-V 18" bar (DCS5000).
They're both fantastic saws.
Echo has discontinued the 58v volt line and the 56v batteries are quite a bit smaller, which makes it an upgrade. I would only consider the 56v line going forward, as the 58v batteries have mixed reviews (although I have 3 and have never had an issue). I will use the 58v (trimmer, edger, and chainsaw) until they don't work and then I'll move exclusively to the 56v.
In the 56v line, I have the chainsaw and the lawnmower. The mower is fantastic, cuts very nice, easy to use.
I have both the 12" dewalt and the 16" dewalt chainsaws (20v/60v) as well. The 12" serves a different purpose than the Echos, and I just got the 16" 60v dewalt saw and haven't yet used it. The Dewalt saws are great for what I use it for (cutting small limbs, bbq wood).
Any more questions?