I wonder what are the advantages and disadvantage of using NTFS compression. I wonder does it slow things down a lot?
There is a decent page on Microsoft's site explaining the pros and cons.
Basically, there is a performance penalty so don't use it on files and folders you access often. A small delay incurred infrequently is probably ok for most people.
I suppose that if you are getting desparate for disk space then you may be willing to accept the performance hit and use it for more frequently used stuff.
The actual performance hit will depend very much on your machine, if you are getting short of space try it, otherwise I wouldn't bother.