For me with my Galaxy Nexus (16GB, 13.33GB after OS), I have 5.7GB in in Apps, 2.84GB in Music (about 25~ mp3 albums, give or take), another 2.2GB in miscellaneous, with about 2GB left over.
So it's enough to get by, especially if you have good data. For me, I don't have good data (I'd call it fair at best) and my coverage area is relatively limited (I'm with WIND Mobile). So for me, onboard storage is still very important. I've mentioned before that USB OTG/Flash Drive is not a very efficient nor convenient solution.