Ok, got both my 2 Dot and 1 Tap units.
The fidelity on the Dot is nowhere near that of a full-sized Echo. 'Tinny' about describes it. If you were underwhelmed by audio from the regular Echo (compared to say, a Sonos) then you'll definitely find the Dot lacking. Enough such that my wife insisted on an Echo instead of the Dot for her office at work.
The Dot appears to respond to voice commands the same way as a regular Echo. It uses a micro-USB connector for power and has a 3.5mm socket for audio output. The box included micro-USB cable and transformer, along with a 6' stereo 3.5mm cord for audio. The sockets are flush with the back of the Dot, whereas the full-size Echo tucks the lone power socket underneath itself.
I set the Dot up using external speakers. It sends ALL audio out. It'd be interesting to have the option to play some things through it's own speaker vs music out. I don't know what it supports, but suspect it's a simple either-or; all or nothing audio jack. It does deliver stereo output. Can't speak to the quality of the codec but it's on-par with the same output from PC playing the same source material.
The Tap is interesting. It sets up the same as a regular Echo and Dot. It's about 1/2 the size of a regular Echo. It fits well in the hand. It uses a base for charging and has a set of ring contacts on the bottom for this. The fidelity is better than the Dot, of course, but slightly less than the regular Echo. I set the Tap and an Echo on the same counter and had them play the same song. The Echo had better fidelity but not as dramatically better than a Dot. I could see the Tap being useful in the same way as a lot of other Bluetooth-connectable speakers, with the added perk of push-to-talk Echo functionality when there's an Internet connection available.
There's a rubberized case available for the Tap. It covers the bottom of the Tap such that you use the back microUSB port to charge it instead of the cradle. I'd have preferred if it used wireless charging and that it worked through a case. There's likely some 'good reason' for going with wired charging for the Tap but it's darned convenient on my Android devices.