I haven't yet, but my summer / fall is going to be spent working with one of my profs to set up a lab with CUDA workstations and a Tesla. If you're interested in GPGPU programming, CUDA works with any system with a GeForce 8XXX or higher GPU. So ~$150 for a 9800GTX will get you 128 equivalent cores, which you can easily double at any time by running SLI.

What I'm interested in is seeing how OpenCL plays out, as Khronos seems to have the entire industry behind them (with the exception of Microsoft).