The OpenCL implementation provided by Apple is less than ideal. It lacks a number of optimizations that are provided by, for example, AMD's OpenCL driver packages for Linux and Windows.

That, coupled with the wretched GPU options available on the Mac Pro, make the current stack not really cost effective. (I think a 5770 is still $250.)

However, it is better than CPU-only.

I've found slaving a cheap Linux box with expensive GPUs to be a much more gratifying experience.

I've done most of my development on a Mac, but I keep finding many bugs in Apple's OpenCL implementation. Intel's seems to follow the standard most closely, which is useful for keeping me honest.

OpenCL is used to JIT OpenGL calls based on the hardware. Its magic.

Are GLSL and OpenCL equally capable?

Not exactly sure what you're asking, but GLSL and OpenCL are for two different domains: GLSL for graphics and OpenCL for calculations that are not intended to show up as graphics.

They both run on the same hardware, but I don't have enough (or really any) experience with OpenCL to say if it feels more or less powerful than GLSL.

Edit: It turns out that I have no OpenCL hardware support (mid 2011 MBA), so I guess the hardware isn't necessarily completely similar (i.e. it can support GLSL and not OpenCL)

This isn't entirely true. As of OpenGL 4.3 (latest spec), OpenGL adds compute shaders as well. Their primary usage is to make interopt with the graphics pipeline less painful than it would be with OpenCL, but in theory you could use OpenGL (or DX11 on Windows) compute shaders for non-graphics tasks, and they'd have roughly the same capabilities as CUDA or OpenCL.