This is exciting news, they said they want to keep it up to date with current releases of the engine so that means CryEngine
3 for all you lovely people to play with.
I will have to check what they do with the licence as to regards to selling your creations, if it is any better than UDK then i'd consider switching my project over to it, not that I'm not tempted already. I can't see it being completely free to sell all creations created from the engine and I have no idea what any limitations on the engine would be, if at all.
The UDK doesn't allow you to edit the c++ source code, but you can freely edit the Uscript files concerning the engine, I have no idea what they plan to do regarding any of this, but I believe its the only way they can realistically compete with UDK and Unity. If anyone who knows Sandbox well enough, does it actually have any internal scripting ability?
If the opportunity is there to completely rewrite the engine (in a UT fashion) then I wouldn't hesitate in switching over, but if it's a simple affair of just creating missions and levels for the existing Crysis engine, then it's just a glorified level editor.
We'll see what happens when more gets announced about this.