My opinion is that they offer significant improvements across the board for the audio signal.
Which brand and model? I suppose it depends upon what the budget is and the quality of the rest of the system. Some solid budget names are Parasound, Musical Fidelity, Pro-Ject, Schiit, Rega, and maybe NAD. In that I am a fan of vinyl, but never invested in it this is passed down from what I read online and in the high end rags. I personally know the owner and engineer at Schiit. Silly name, but a great product line. There is an amusing story behind the name.
What speakers and amplification are you using?
I am using a set of Wilson Watt Puppies and a Jeff Rowland receiver. My front end is an OPPO universal opitical player.