R.U.B.E is very fast, but feels very low level. I don’t like that editing different elements requires mode switches and that some functionality is only bound to keyboard shortcuts, but that probably makes it very fast to use once you’ve learned them. Some of the editing keys feel familiar from Blender, so a Blender user may be at an advantage. R.U.B.E embeds AngelScript as a scripting language to extend and customise the editor.
It would be useful to be able to organise a scene into different layers. Similar to Inkscape I am missing some kind of library to store reusable gameplay elements, but this could probably be implemented using the scripting language.
The file format is designed around the Box2D runtime, but with a bit of work it is possible to use it in a different engine. It is a JSON document that can be easily accessed from many programming languages.
Update: For some reason the spambots like this post, so I disabled comment posting. Sorry about that!