Arcane Torrent is 210% weapon damage, and with the Disruption rune jumps to 241.5% (210% * 1.15) weapon damage and costs 16 mana per second to channel for area of effect. That works out to 15 damage per arcane spent.
Let's compare that to...
Arcane Orb (post 1.0.7 buff) is 200% weapon damage, 20 arcane with the Tap the Source rune, which works out to 10 damage per arcane.
Disintegrate with Intensify is 221% weapon damage at 18 Arcane per second, at 12% damage per arcane.
Ray of Frost is equal cost, higher damage, but limited to a single target.
Meteor with Molten Impact is ( 390% + 90% = 480% ) @ 50 Arcane is 9.6% damage per arcane.
As you can see, Arcane Torrent is very mana efficient and good for single targets and compact groups. It does have a .5 second delay, which takes some getting used to, and yes it probably could use the ability to shoot over walls.
However, it's not a crap skill at all, IMO, and is actually one of the best wizard mana-dump DPS skills.
So if you're dumping a full mana pool of 100 arcane on a single target, in order of effect: Arcane Torrent nets you 1500% damage, Disintegrate does 1200% damage, Arcane Orb does 1000% damage, and meteor does 960% damage.
Of course each of those abilities has different shapes / AOE effects, so whatever suits your playstyle. But if I'm suiting up for facing an Inferno champ 1v1 with my toon on the line, I'll take Arcane Torrent.
On a side note, post 1.0.7 Blizzard with Snowbound rune jumping to 510% damage looks like it might be sick from a damage and mana efficiency perspective. 510% / 20 = 25.5% damage per arcane.