PS Audio PremierI own two Premiers, one of which unfortunately just went on the blink after a few months of operation. It is at PS Audio being repaired -- I will be interested to see what they say origin of the problem was. It started putting out 90 volts instead of 120 at over 12% distortion (by its own meter, which I checked with my own multimeter). My other Premier is a year old with no problems.
Generally, I agree with Markl's comments. The Premier lowers the amount of noise in the music you hear (and video you watch) by giving your equipment better quality electricity to work with. Lowering the "noise floor" in your music is extremely important as it allows you to hear more of what is on the recording. It will make a difference in every single piece of music you listen to, with better results, of course, on better recordings (because there is more detail -- such as hall ambience -- to begin with). Unfortunately, lowering the noise floor in a system is almost always expensive (the other component I have purchased recently that significantly lowers the noise floor is a preamp by Audio Research). Dedicated electrical supply circuits can (but don't always) help here as well.
All other things being equal, one reason audio equipment usually sounds better at night is that there are fewer sources of RFI and EMI using the power lines as transmission antennae.
I also own an Equi=Tech Q isolation transformer which is about the same price as the Premier and has far fewer electronic parts to go bad (the Premier is comparable to a transistor based power amp in complexity and failure modes). I am not sure that the Q is 100% as good in lowering the noise floor as the Premier is at its best (the Q won't reduce harmonic distortion in the power it outputs, so maybe the Q is only 90% as good) but in my experience the Equi=Tech products are far more reliable and more foolproof than the PS Audio products. At the margin, for me, it is a toss up between slightly better peformance for the Premier but way better reliability for the Q. PS Audio won the last round, so I have two Premiers, but after the problem I just experienced with my second Premier, my next high end power unit will probably be an Equi-Tech again.
For anyone who doubts that these things work there is an easy test: compare the picture on a TV plugged into a wall to the picture on the same TV plugged into a Premier or Equi-Tech Q. It is usually easy for even non-hobbyists to see the difference because most people are far more sensitive to visual changes than audio changes.
Whether the degree of improvement is worth it is up to you, of course. Like most people here I am a hobbyist and have trained ears, so it is worth it to me. On the other hand, though I can tell the difference between a $50 wine and a $15 wine -- the $50 wine really is better -- I personally buy the $15 (or preferably $10-12) wine, so "better" in this case is not "worth it."
One alternative worthy of investigating for those who don't want to (or can't) put a lot of money into a power conditioner is the APC H15 power conditioner. APC is a expert in power conditioning and supplies products, e.g., for data centers, to large, demanding customers such as IBM.
I bought an APC H15 for a small home theater I am putting together for my daughter and she was so pleased with the results I bought another one for use with some of my own video components. You can get the H15 for $400-450 from several very reliable Internet electronics houses (B&H, Vanns) and in addition to EMI and RFI filtering it does under-voltage and over-voltage correction, which is nice to have. Plus it is very cool looking. My 25 year old daughter was quite pleased with its appearance in her equipment stack! And it helped her 52" Samsung 650 perform very well in a high rise apartment in an urban environment (where you would expect to have lots of noise on the power line).