Originally Posted by FMW
I think we are into opinion here and not test results. If you can make an MP3 that is audibly indistinguishable from the original CD - and you certainly can - what would make FLAC or AAC or anything else better. Different, perhaps, but better? I think people tend to equate MP3 with low bit rate MP3. High bit rate MP3's are audibly indistinguishable in a blind test from WAV.
The main benefit of FLAC, aside from ensuring that your music has no lossy artifacts, is that it saves you the trouble of re-ripping your CDs if you want to encode them to multiple lossy formats over the course of time. People often seem to rip their CDs to 320 kbps MP3, only to decide later that they want to save more space. Since transcoding MP3 -> MP3 is a sub-optimal solution, encoding MP3s from FLAC files is more convenient than extracting the lossless audio from your CDs a second time.
As for AAC, in a very general sense, MP3 can become transparent at ~192 kbps, whereas the improvements in the AAC format allow it to reach transparency at a lower bitrate of perhaps ~150 kbps. As such, AAC can save even more space than MP3 without sacrificing sound quality, and depending on the individual listener's tolerance for artifacts, he might be able to use an even lower bitrate without hearing any artifacts.
It's absolutely correct that if you're using a sufficiently high VBR quality setting that generates bitrates in excess of 200 kbps, there is no difference between the fidelity of MP3 and AAC, and that's exactly why AAC has failed to supplant MP3, despite being the technically superior format. The fact that AAC doesn't even gain a 2x compression improvement over MP3 means that its extra efficiency fails to outweigh the benefits of MP3's wider compatibility for many people, since everything but your toaster can play MP3 files.
As for double-blind testing, there is more than one type of test you can conduct to evaluate the quality of lossy codecs. An ABX test lets you compare a lossy file to the lossless original to determine if the lossy file is transparent, whereas an ABC/HR test allows you to score multiple non-transparent lossy codecs against each other to objectively rank their sound quality. For example, this ABC/HR test
compares Nero AAC, Apple, AAC, FAAC, and LAME MP3 at 64, 96, 128, and 192 kbps. None of the lower bitrates reached transparency on the test samples, but the test ranked Apple AAC > Nero AAC > LAME MP3 > FAAC in terms of quality, and AAC showed a clear advantage over MP3 in terms of quality at mid-range bitrates.