That depends a lot on what kind of a weapon you use it with and what else you do in combination with it. At high levels, the spell becomes very very good.
For instance, a +4 (through greater magic weapon) holy wounding greatsword thrown by a level 16+fighter/mage with a 24 int will do a base of 2d6+4+2d6 (to evil foes)+1 point of constitution+int (and if you get 1.5 Int with a two-handed weapon, that's 10 points). So, that's a base of 21 points of damage, 1 point of con, and another 7 points against an evil foe at full attack bonus. Assuming an evil foe, and a 16 hit die foe, that's as good as a lightning bolt but with no spell resistance, no energy resistance, and no saving throw.
Not bad for a 2nd level spell. To get comparable damage with a mediocre int like 24 (figuring in saving throws), you would need to turn to 5th level spells like arc of lightning.
But, it gets better. You can use arcane strike with it. Since arcane strike is a free action rather than a swift action, nothing stops you from casting another swift spell in the round. Still, just taking things one at a time let's assume a moderate arcane strike like a 5th level slot. Assuming further that the +27 odd (+13 BAB, +7 Int, +4 weapon, +2 morale (heroism), +1 weapon focus) attack bonus that the character is likely to have for the attack roll is already sufficient to hit reliably, we'll trade the attack bonus for damage with power attack. Now, that's going to add 10 damage from Power Attack and an additional 12.5 damage from arcane strike dice, bringing the total to 43.5 plus 1 con and 7 vs evil foes. Against the same 16 HD foes, that's about 58 points of damage which is more than the arc of lightning would net you.
But, remember that you still have a swift action left. For a 6th level spell slot, you can drop a quickened whirling blade and do it again. 116 odd points of damage for a 2nd, 5th, and 6th level slot. Not bad.
But, better still, imagine that you found a way to have fires of purity active at the time (contingency perhaps). Now you get two 73 damage attacks on everyone in the line.
But it gets better. If you add wraithstrike to it (which you really shouldn't because wraithstrike is heinously broken), you can crank the Power Attack up to max. Without Arcane Strike, that brings the damage up to 52 points of damage in a line for a pair of second level spells. And, what's worse, you can extend wraithstrike and drop a pair of whirling blades in the next round as well with full power attack and with a hefty spell on the arcane strike. Dropping an 7th level spell ups the damage to 69.5 per attack for a total of 139 damage in a line on the second pair of attacks. The cost (3rd level slot the round before, 2nd level slot, 6th level slot, and 7th level slot in the round used) is high, but the damage output exceeds the area effect damage you can do, even with 9th level slots.
Darklone said:
.... wraithstrike ....
I like Whirling Blade but I think the spell is kinda weak.