Vala is an object-oriented programming language with a self-hosting compiler that generates C code and uses the GObject system.
Ranked in these QuestionsQuestion Ranking
Pro Easy Syntax
Vala is syntactically similar to C# and includes several features such as: anonymous functions, signals, properties, generics, assisted memory management, exception handling, type inference, and foreach statements
Pro Rich GTK Support
Vala developed by Gnome Foundation.
Underneath there is C. It makes some stuff easier to write than it would be with plain C code.
Pro Well-integrated with C language
Vala itself is compiled to C, therefore it can use the vast ecosystem of C language, with least effort.
Pro Good documentation
There is Valadoc and most of the libraries have Devhelp packages.
Pro Stable ABI
You can write a library with ABI stability.
Pro A general purpose language
Vala is compiled to C and only requires GLib - or even nothing (posix profile)
Pro C compatible API (and ABI)
C programs can use libraries written in Vala, and vice versa.
Con Very limited adoption
Vala isn't used in areas other than GTK development, and no major business has adopted the language.
Con Not a general purpose language
Can't use Vala in multiple software development areas.
Con Very limited tooling
Though using Gnome Builder, you can debug it easily.
Con Heavy reliance on GObject
Heavy reliance on GObject types, although Vala can be used without GObject. (posix profile)
Con Not well documented
It already has tons of documentation, even a book about Vala.https://leanpub.com/vala https://wiki.gnome.org/Projects/Vala/Documentation
Con Very few resources allocated to the language
Only a single person is working on Vala full time. Gnome Foundation isn't allocating sufficient resources to properly maintain Vala.
Con No stable version
No stable version of Vala released for over a decade so far. Means that Vala isn't production ready.
Con A very naïve approach to the real problem
Vala just adds more complexity to the game. No real benefits in using a 'programming language emulator' for UI code, you may think it adds to productivity but it's just a way to make the problem last longer, reinventing the wheel everytime and worse.