70% performance (more likely closer to 65%) for 53% the price seems reasonable until you realize that highest end halo parts are meant to have terrible bang for the buck. High end price/performance in the midrange just simply isn't compelling. Also, we've seen 470s for under $300 and 5850s for $250 recently.
As far as OCing -- the 480s (and 470s and 5850s) can be OCd too. I don't think we'll see Intel-like 100% headroom built into these chips, however.
These parts are meant to compete with the 5770 and 5830. And like their big brothers competing with the 5850 and 5870 I have no doubt they'll outperform the half-sized AMD competition by similar margins to their bigger brethren. The question being: is 5-10% more performance worth the extra $? And the answer will depend on what the extra $ is. At $240 these bad boys are a non-starter, much like the 465 they will replace and the 5830 they are meant to compete with. For $170 they'll rule the roost.