I think it is a really nice concept. Unlike Objective-C, it only needs a C compiler so is extremely portable.
The slight issue is that it uses a garbage collector that whilst is pretty portable the way it is implemented (obtaining pointer location of volatile stack variables), it still makes a few assumptions about the stack that isn't 100% portable.
I have spoken to the developer (Daniel Holden) whilst I was writing something similar (just to avoid memory issues) and he was a great help. He also has some really interesting work on 3D graphics.
All in all, just as he mentions on the site, libcello is great... but probably wouldn't use it in production in case anything goes wrong because it is quite non-standard.