Proper double-blind tests using high-quality equipment and experienced listeners have been done, and the usual conclusion is that 320kb/s FBR or 256kb/s VBR MP3s can't be reliably distinguished from the originals even with critical material.
I've done the same test myself using high-quality headphones and the studio masters of the band I used to play with -- which do include multiple instruments with a lot of high-frequency content, cymbals, snare etc. -- and came to the same conclusion. So I'm happy to use 256k VBR -- with the LAME encoder, not all encoders are as good!
Anyone who makes claims like "the difference is night and day" without showing that they can reliably tell the difference in a double-blind test -- which they can easily do themselves! -- should be put into the same box as those who promote gold-plated mains cables and green pens for colouring CDs.
And don't get your information about this by reading hi-fi websites, most of them (with a few notable exceptions) are as guilty of promoting the same audio bulls*it as the gullible audiophiles who buy it...