In way of support of Unikron Software, my own $0.02 is I agree with micky. I bought 2D Toolkit
after the release of Unity 4.3 and its native 2D features. I did this because while Unity's new box2d physics and better 2D integration into the editor interested me, 2D Toolkit's features completed the package and sold it. Better GUI tools, tilemaps, static sprite batching, fonts, platform dependent sprites, pixel perfect camera. All very important tools to me that Unity doesn't yet support, or not as well as I'd like.
Plus, looking through these forums before buying, it made me confident that Unikron Software will continue build 2D Toolkit as an extension of Unity's native functionality, rather than it becoming an obsolete plugin.
it's all well and good to play around with a plugin but when you have a project with millions of lines of code the last thing you need is for your builds to break because you want to upgrade to a newer version that has features you need.
My own personal view on this is you should never upgrade mid-production unless you absolutely have to, which also means you shouldn't use a tool that doesn't have the features you need when you start. If an upgrade is deemed worthwhile, significant project time should be devoted to making the upgrade. It's nice to hear Unikron takes API stability so seriously though.