Interestingly, design-wise it's very similar looking to the XL battery pack* i had for my old HTC desire. Even i thought it was ugly at the time, but functionality > looks IMHO.
Have a similar XL battery** for my current samsung S3 (4600mAh instead of the default 2100mAh), but it's better in that the back is the same thickness throughout rather than an oddly-shaped lump, which is better both for looks and ergonomics.
I'll definitely always go for the XL battery option over the fixed battery+external battery cludge though. I hate the idea of having the most likely component in your phone that can wear-out/fail as non-replaceable without considerable hassle...
* https://mugen.co/htc/htc-desire/muge...n-3200mah.html
** https://mugen.co/mugen-power-4600mah...tery-door.html