GGI is currently under development. The list of supported chipsets is slowly growing, the API has stabilized. Support for more complex drawing operations than circles has only just begun.
If you want to use the LibGGI in your application, in my opininon you can safely start. The user API is not likely to change very much in the near future (you never know, though). I use it as a very convenient method for graphics in X now.
If you want to develop a kernel driver, be aware that the kernel interface is still a (albeit slowly) moving target. If you don't fear that, you are more than welcome!