Personally I don't think it is, but some people swear by it. Have a look at your shutter speeds that you're getting with your current lens, if it's not very low, I wouldn't bother.
Chris
I'd put the extra to a nice prime -- in Nikon land that'd be a 35/1.8 or similar.If you have it then you can choose not to use it if you don't need it.
If you don't have it then you have no choice...
As said above think of a situation that you find yourself in frequently enough to justify the extra cost. If you will use it in plenty of low light situations then go for it, if not is the extra flexibility it offers worth the extra cost?
Recently bought the Sigma 17-70 with IS as my ultimate, flexible kit lens replacement.
IS is there if I need it .
How are you finding the 17-70 with IS?
My dad has one and he says it's exactly the same as the old one (which is for sale) but better IQ especially when zoomed in, i.e you can see where the IS has taken affect.