I've only done some casual listening for about 10 mins in a Mac dealership (there are no official Apple stores around where I live), but they sound pretty alright, particularly as far as monitor speakers go. All of Apple's more recent stuff (as in the last 5ish years or so) couple tuned resonance chambers to each speaker driver, which make even smaller speakers sound pretty nice and loud. My 13" Macbook Pro has a tiny built-in speaker, and it's not a hi-fi device by any means, but thanks to the resonance chamber it's at least useable.
The amp in the ACD is rated (by Apple, so not sure if it's RMS power or what the hell it might be) at 49W, so they are some decent sized cans. My last monitor with speakers, a BenQ 24" display, had tiny speakers that were pathetic and sounded absolutely terrible. Not in the same league as the ACD, or even same ballgame really.
Not saying the ACD can replace a decent set of standalone speakers however; it probably can't simply because the sound coming from the monitor is unfortunately aimed downwards, at the desk. This simply for aesthetic Apple reasons, to not break up the smooth, shiny front bezel with "distracting" speaker grilles...

Also, the stereo separation is limited of course by the width of the monitor itself.
However, if you feel you don't have room for external speakers, or you don't want the extra clutter, the speakers of the ACD will serve you pretty well. They certainly don't sound bad, that's for sure.
