The speaker within the X-mini Capsule Speaker system has a 40mm driver capable of 2.5W output with a claimed playback time of up to 11 hours when fully charged via the micro-USB port.
Having checked the specifications, we were pretty sure that 2.5W output fell well below what we could realistically class as "unbelievably loud". That kind of hype doesn't really help a product, but we were mostly pleased with the general clarity of the X-mini II's audio output. Even if there isn't space to twist the unit fully open the general audio output for a single listener is fine at most sensible volumes. When we cranked up the output from a connected iPhone 3GS on a bass-heavy track we did hit some distortion. You also won't be filling a party room with the output from a single X-mini II.
We only had the one X-mini to test with, so we can't speak to how well the daisy chaining actually works. As for headphones, we can confirm that the X-mini II outputs to headphones, but we're not quite sure why this would be useful, given you've got a speaker right next to you. If you needed just headphones, you could plug them directly into your audio source.
The X-mini IIs don't live up to the "unbelievably loud" hype, but beyond that they're fairly priced for portable speakers, somewhat unique looking and offer up decent but not spectacular audio quality.