Yes, but my point was to say that chances are there won't be enough who can comment in depth on it; and if they did they'd probably be mostly in the negative considering they'd go through making it even bulkier by having a DAC-Amp with it (or as I assume it's not all about making use of the digital output). In any case I did use the headphone out on a friend's iPad and occasionally on my iPad2, and it's mostly for convenience and almost never with music (ie, playing games and I can't let the noise bother other people).
But of course I had to test everything (FLACPlayer and FLAC on my own iPad2, ALAC and AAC on the iPad; mostly jazz and audiophile recordings), and generally the iPad's headphone out works best with earphones like the CX300 and MEE M6; it was horrid with the Westone2 (Triple flange); and almost listenable with the Grado SR80 (small pads) and SR225 (large pads). On the ones that were good for listening, the sound was generally warm but with a few sharp edges on the treble; the Westone just seemed to sound wrong everywhere - bass was non-existent, midrange seemed EQ'd to the point that it almost doesn't sound like what it normally would in any of my other systems; with the Grados it was like listening to an average factory sound system in most modern cars - has many sharp edges, generally a little thin, but it wasn't too far from natural.
Then just for kicks, because my brother was asking why use the USB input and bother with a CCK, I hooked up the iPad to my Meier amp, with the SR80 hooked up - same tonal quality but with a little bit more oomph in the sound, mostly the bass, but that's about it. If anything the sharp edges seemed a little sharper, but I might have cranked it up higher since the Meier distorts at a higher volume past where the iPad does.
Basically, if you're gonna use an external amp, might as well use one with a DAC. If you don't want the extra equipment, especially on the go, get 'safe'-sounding (generally, much more warm than analytical) earphones. I'd recommend the CX300 over the MEE M6, if just between those that I've heard - the M6 still exhibits some of the sharp edges, noticeably more than the CX300. On most internet material though it is crisp and clear enough, like dialogue on Family Guy; or of course explosions on games and movies.