I'm wondering if it's just another double swing(diablo 2 barb), or if I should try to make a character specializing in it.
You Very likely could. Infact I know you could depending on game difficulty(and even then..you could if wanted) etc. Having said that, Spectral Echo is so far better it's almost laughable. It's not AS damaging itself...but if you add your base attack PLUS the echo slash it's more so. It's Faster if you use fast weapons. FAR faster actually. AND if you are using leach HP/mana items it will work with that for the normal attack portion of the ability where as slash will not drain at all unless you use a mod that adds that to special attacks. I personally left it at 1 point and never looked back. It works nice in the beginning though Even with just the base point.
The thing about slash is it's a "One point wonder" (1der), so few people actually throw more than one into it. Also, there is little reason to do so as it becomes less and less mana efficient. However, it IS more damaging than spectral echo when you are dual wielding (it uses both weapon damages, meaning it hits for 300% damage at rank 10, vs 180% for spectral echo) and the width of the arc is much better, and it has an 86% chance of interrupting attacks, which makes it pretty much infinitely better than spectral echo when you're in a situation where it's 1 v many (meaning most of the game). The trouble is, each attack is costing you 15 mana vs leeching mana w/ spectral echo, and if you're playing a build where you are maintaining multiple buffs (frost shield et all), mana is at a premium. Also, against enemies that hit hard, the leech from Spectral Echo can make all the difference in the world.
The other thing to keep in mind is that slash increases in speed the higher your cast speed goes, whereas spectral echo increases in speed the higher your attack speed goes. So, if you stack cast speed (as I do), the speed difference is reduced greatly, whereas if you stack attack speed, spectral echo will be WAY faster per attack, potentially to the point where even dual wielding it does more damage, especially if you have attack speed boosts (such as haste or dervish).
Vs multiple enemies (trash mobs), Slash remains a better option through the entire game. I use it on VH, and it works great. The interrupt chance alone makes it worth it, but the damage and arc are better too. Just decide whether you want a 6 mana attack for 160% damage and 50% interrupt (plus 9 more skill points) or a 15 mana attack for 300% damage with a 80% interrupt, but either way if you dual wield, use it.
Vs single enemies (champions and bosses), as well as against interrupt immune foes, Spectral Echo is a better option, especially if you have attack speed boosters. It scales linearly, and therefore if you're going to use it at all, you may as well max it. As it's an augment to auto attack, though, it's flat out better than using auto attacks for all situations in which you would normally do so, from the first point onwards.
My strategy? Use and max both. I personally run w/ the normal buffs (spectral armor, entropic aura, frost shield), stampede into foes (another 1der) and slash them all to smithereens. When I get in trouble I Chain Vortex my enemies, and against "tough" foes (champions and bosses) I pop spectral echo and spectral decay and cut them to pieces. My point? You have a lot of tools at your disposal as a Destroyer. Why not use them all? And mainly, yes, slash is viable endgame, just not optimal vs Ordrak