It depends a huge amount on what you're listening to. For about 90% of my music, I can't tell the difference between the original CD and 128kb/s MP3. A few things have noticeable artefacts that don't go away no matter how high you put the bitrate. Substitute 128kb/s AAC and that changes to over 95%. At 256kb/s AAC, I can't tell the difference for anything I own, but I've heard some recordings that hit pathological cases in the algorithms used for AAC and sound terrible at any bit rate (usually orchestral pieces with a single voice and only for short samples). With FLAC, you can 100% reconstruct the original, bit for bit, so you won't suffer from any unfortunate coincidence between your choice of music and the CODEC of choice.
The big advantage of a lossless compression though is for recompressing. For a long time I had a DVD player connected to my living room speakers that could play back MP3s, but not AAC. If I wanted to burn a CD-RW or DVD+RW to play on it, I had to recompress, which usually sounded noticeably worse than if I'd gone straight to MP3 from the source material. If I'd ripped everything as FLAC, then that recompression would not have introduced any new artefacts.