I have only been using C2 for about a year, so transitioning to C3 has been easy for me.
I have a had a couple crashes but LOVE the error message that allows you to save before reloading. Nice touch.
All of this said I have given up on developing with it because of the "preview once, yes, preview twice, no." bug, which means that I have to reload the project after every preview. If I don't, the preview simply doesn't start when I click on the play button (or debug button). With that, it is just too time consuming.
[EDIT: If you go into Menu|Settings and change the drop down "Preview With" from "PopUp" to "Dialog" the preview issue is gone. Hooray! (Yes, I had already turned off blocking popups in chrome, so it isn't that)]
All in all, this is certainly worth 99 bucks/year to me. Especially with native mobile app compilation.
Thanks to Scirra for their hard work!