GLBasic is great for 2D games, but is lacking a lot of functionality for 3D (stuck with OpenGLES 1.1 last time I checked), so lacks most shader functionality. Also, both 2D and 3D have no physics engine. And it is not an OOP language (debatable if this is good / bad!

). Multi-platform support is buggy, but considering only a couple of people are working on it, it is actually pretty good.
But understanding this, it still has some amazing strengths.
It is fast, in my testing, as you mentioned.
It is very easy to learn (my nephew had a platformer style game working with almost no programming experience). Who doesn't love programming in BASIC?!
Knowledgeable and helpful forum support (although fairly quiet recently)!
I'd still be using it if it was OOP, had physics, and an up-to-date OpenGL.
I was (half joking) suggesting a rewrite if it could fix the above issues. And not for competing with the other engines, but as a learning experience. (Rather than trying to patch the existing GLBasic code.) Ha, ya it would be difficult - I wouldn't know where to begin!