I think it depends on what kind of levels you're trying to design.
The problem with AGk... I mean to say, the challenge with AppGameKit is texturing the different parts of the level. There's no straight forward way of linking the model with the images for the textures, you have to do it with code. But it is do able.
The way I did it for Hover Car (see link in signature) was to set up an external data file that linked everything together. It's possible to make a level editor in AppGameKit to create the data file or you could just construct it manually (using notepad, for instance). I don't recommend doing it manually if there's a lot of details but it's a good way to get started.
Actually creating the level again depends on the type of level. I mainly do interiors so just use Blender and then export it.
This is my current "big" AppGameKit project (it's changed quite a bit recently).
There are no textures, yet, but the map is split into a number of sections and coloured differently, to give contrast. It was all modelled in Blender, split up and exported as a number of obj files, it's made up half a dozen bits. The obj's are then loaded and coloured individually.
It's all controlled by an external human readable text file so I can edit that, or create a completely different level, and not have to change the code.
With a bit of thought the code for loading a level in can be very concise, it all just has to be set up in a logical manner.
