I think the speakers in the 27" sounds very good in relation to its size. It's very crisp and clear, and there is even some slight amount of bass to it.
The reason I ultimately decided to go external was because of a weird issue I'm experiencing with it.
I'm finding speaker volume tends to fluctuate randomly if I adjust volume level in apps individually (such as iTunes' volume slider) instead of using the system's universal volume adjuster. When app volumes are near max or maxed, volume never fluctuates regardless of system volume. When app volumes are adjusted midway and below, it starts to fluctuate (again, regardless of system volume). It's such a strange issue that I thought something was wrong with my iMac hardware. A visit to an Apple Store and Best Buy reassured me it's likely a software bug, as all the iMacs on display and even the MacBooks had the same problem. But again, most people would likely never notice this as I'm quite sensitive to noise.
Also, speakers sounds terrible under Windows. They sound like they are muffled and covered up. However, there is zero volume fluctuation regardless of system or app volume. That tells me the speakers are greatly enhanced under OS X with software running in the background.
I went with the Swans M10. Beautiful looking speakers and great sounding. I waited almost a year for them to be back in stock, but you can finally buy them right now on
Audio Insider! I also have the Klipsch 2.1 ProMedias and they sound just as good as the Swans (although they are somewhat ugly).
Oh yeah, I tried playing the speakers on the 21" iMacs on display and they sounded bad compared to the 27". The volume on it just didn't get very loud at all.