1/2" is not that big of a deal.

the only thing that it'd do is wear off your left-rear tire a little faster, but if you keep switching it w/ right-rear it would be good. you shouldn't really feel the croocked ness. (like every time you get an oil change, just tell them to switch the back tires)
i'd say its totally worth it. you'll probably pay about 200-300 dollars to get the frame pulled, but its up to you. like i said i wouldn't really care that much about it.