How good it is depends on how many dice you roll and how big they are. Big dice make it more valuable because you have more variance, more dice make it less valuable because the more dice you roll the more likely your roll will regress to the mean.
Average damage on a single d6 goes from 3.5 to 4.47, on a d10 it goes from 5.5 to 7.15. A d12 goes from 6.5 to 8.48. I haven't crunched the numbers on 2d6 (I'm sure proper stasticians have a more elegant technique then what I've been doing), but I'd expect it to be worth less in that case.
Edit: Okay, because I like a challenge and I have a masochistic streak, I did the math, and I figure 2d6 goes from 7 average damage to 8.37 with savage attack.
So there you have it, Savage Attack is worth around +1 to +2 damage. Not super awesome, but if damage is all you care about...