i say it depends on what gear you listen to, and how much of a listener you are.

what kills me with lower bitrates, and mp3 files in general, is that high-end detail garbles up. you don´t notice it when it´s all loud and stuff, but when there´s a quiet part, and the singer whispers, or there are cymbals ringing, i hear it way too well. annoys me to no end!

however when i´m riding the bus, and i´ve put on isolating headphones, and i´m listening through my ipod, the finest details aren´t discernable enough to matter. and so mp3 files are just fine.

for normal listening, high bitrate mp3´s are just fine. lower bitrate mp3´s however, fail to impress. i can notice the difference quite far up in the bitrate ladder. the thing is though, you have to go far between different bitrates to notice it. smaller jumps are unnoticeable.

just my humble opinion