In my experience, many - but not all - CDs can be improved by being copied to CD-R. Alesis Masterlink does an excellent job, but I've had equally good results using plain old Roxio on a good PC. For CDs which were produced with great care (e.g. JVC XRCD, Reference Recordings) there is usually no improvement. For others, it may be minimal or very significant. I have yet to make a CD-R copy which sounded worse than the original.
Robert Harley hears essentially the same thing. Neither of us KNOWS why. Our best mutual guess is that the CD-R copies are clocked more accurately than the originals, reducing jitter. My guess is that these errors are introduced during then physical production of the disc rather than existing in the digital master. (I also find the discussion of reading stamped vs dyed discs interesting.)
BTW, I've tried several types of CD-R media. Verbatim 'Vinyl CD-R' s sound the best to me.
One last thing... Digital copies do not degrade with each generation, as analog copies do. Reading & writing to magnetic disc is VERY accurate... or computers wouldn't work very well, and couldn't be trusted for business, much less scientific calculation. Even on PC, a dropped bit is extremely rare. (Too bad the operating systems & software aren't as reliable!)