Linux as A Musician's OS?
Last time I tried CSound, I couldn't really get into it, but Pd is nice. The learning curve is admittedly something of a learning cliff, but I think the interface it presents -- a blank canvas on which one draws networks of operators, subpatches, unit generators, etc. -- is close to ideal for this kind of work.
I've still found it to be too much work to build, say, an entire softsynth in Pd (although people have done so), but I've had a lot of luck creating nifty effects boxes, delay units, and audio/data gadgets. Combining it Pd with LADSPA plugins has been especially effective.