I got the early upgrade at a low price, so I took the step from Izotope RX 1 to RX 2 Advanced, at the about the same price as it would cost to buy Izotope RX 2 from the start.
I really like the adaptive noise reduction, it does really help when the background noise changes, and gets the job done a little quicker. It sounds better, though, when unchecking the adaptive noise, but then you have to spend more time learning the noise, especially if the background noise changes. It is all a matter of quality versus time. Generally the denoiser just sounds better, and the new D mode for offline noise reduction sounds even better than the old C mode (which is still there too)
The decrackler is a new plugin in the RX 2 package, and it can remove some of the distortion the declipper can't remove. It also removes a little bit of cloth rustle.
In the advanced version there is also the multi-resolution option in some plugins, it also sounds clearer and more transparent with less artifacts.
If you can get a cheap update at a campaign price, I would say upgrading is a very good idea, it all sounds just a little bit better, there are more features and the overall user experience is better.