Since everyone is asking, here's a quick test.
I took music from a CD converted it to a 256 kbps AAC file.
Listen here. (2.29 MB)
Then THAT file was converted to a 128 kbps AAC file.
Listen here. (1.17 MB)
(So this is what you hear if you double-compressed your files onto your iPod.)
So that's what you get. Take a listen and decide if it's right for you. You really can't ask other people what they think. Do YOU like that or not?
As for me, my speakers don't really let me hear a difference between those 2 files. Your experiences may vary!