Frugal for Life had a poll posted a while back that asked if you washed your shower curtain.
The results showed that 61% of respondants do in fact wash...I am a non washer! I do re-use them, unless they are just too gross for words!
I usually buy the cheap ones (I don't spend more than $5) & when it gets too gross to stand to be around anymore (or more than 4 rings have pulled out, usually happens first) I get a new one. If it is still in good shape, no tears, I'll rinse it with the hose outside & use it for "garage type stuff" paint tarp, etc. They are great under kids when they paint or craft! I do the same with vinyl table cloths.
I have found that spending a little more $ ($5 vs. $1) & getting the mildew resistant style is worth it, the curatin is heavier, lays better & more likely to have metal gromets for the rings to go through. The last one I had lasted twice as long as the $1 one I had before it. And it was still in good condition for re-using. I used it under my last outdoor paint project...
I AM going to upgrade to metal rings, the plastic ones break too easily. I've broken new ones installing them...grrr!!!