The Pi's Broadcom IV GPU is itself a native OpenGLES2.0 system, and it is a fine programmable pipeline API though only a 1st gen version with GLSL1.0 and vertex and fragment shaders. Further more its only a dual core GPU so while it can do a lot, it can't always do it fast.
Though the Pi does currently offer a really good version of OpenGL, you should bear in mind it is going to emulate some functions on the CPU Client side and convert them to OpenGLES2.0 so that the Broadcom GPU can work with it. So while the OpenGL version will provide an appealing and familiar API, which I think is quite comprehensive, your Raspberry is not going to operate at full speed on it.
Very old computer game programmer, now teaching very young computer game programmers, some very bad habits.
Wrote some book about coding Pi's and SBC's, it's out now...go get it!