Well, Joe, it's all about pros and cons.
With OpenGL, you pretty much always get the full use of the hardware right away, but may have to wait for standards. Sometimes the standards are available right away (ARB_fragment_program was available at about the same time as DX9, for example), sometimes they aren't. It all depends upon how much the vendors cooperate. You don't always have to wait for the ARB, either, as you could simply have a standard agreed upon by two parties. For example, nVidia currently uses a few GL_ATI extensions in the GeForce 6800 series.
This can make coding take a bit more time, but considering that OpenGL is already easier to code in, I don't think it's that much of a problem. It also has the added benefit of letting many standards percolate through the many vendors, often leading to a much better implementation than Microsoft.
DirectX has the benefit of putting a standard out the door right away, but it's often not that great. For example, HLSL really should have been what GLSL is.