If you are really into developing for Apple, $99/year is not that bad. Still, the free-as-in-beer Xcode attracted those curious with the platform. Don't forget those who can't set up a store account (Brazil was on this list until not that long ago) will not be able to even try it.
It's certainly a lot cheaper than ~$3000 /year (MSDN?), or ~$4000 /year (IBM?) or ~$3600 /year (Lispworks?).
In the grand scheme of things, Apple's tools and developer membership schemes are fairly generous. It's still making a big step from free to non-free as others have already said. I'm not going to rush to upgrade.
