If you are buying one, the question is whether or not paying half for a camera that gets you 90% there is worth it. OIS will always reduce in some way, so if it turns a non-useable piece of footage into something passable, that is a win. Some would say that 10% is worth it. Electronic image stabilization has come a very long way in recent times.
I do not think that the newest GoPro action camera is a reasonable price. I also don't think that the features are different enough from older versions to justify such a price hike. I think that the company is taking advantage of their recent popularity by trying to get users to buy more.
When a new product comes out, it is usually very expensive. Even a cam corder used to be very expensive. So was a VCR. Over time, GoPro and other companies will learn how to make the same technology for less. Until then, there are older models that provide a similar experience for less cost.
No, it is not worth the price tag. All new electronics are always overpriced when they are first released. Once the price comes down after a few months or a year of being released, it will be worth the price tag. This always happens with electronics, and the owners profit from the people that have to have things when they first come out.