I was able to get my hands on the back cover for the new Mugen battery (back cover only, not the battery itself). They definitely had the right idea with its design and ZeroLemon should take notes. With the rounded back, it doesn't feel nearly as bulky as the ZL even though it's only a slightly smaller battery (9300 vs 10000). Mugen proves that you can have a massive battery without turning your phone into a brick.
Material used is a textured plastic so, while it's not necessarily "grippy", it's not as slippery as the metal back that came with the phone. This should make it less accident prone, but I would have preferred some sort of soft touch, grippy back that Motorola has been known to utilize. With that being said, it's definitely a huge improvement over Mugen's LG G3 battery cover which was an extremely slick plastic.
However, I don't feel that this is exactly the best quality plastic. It feels slightly cheap, but not terribly so. The ZeroLemon definitely feels more sturdy and solid, but that's to be expected from a brick. I would have been happier if this had a soft touch, grippy back that some Moto devices feature or like the Nexus 5 and 7.
On one hand, I think I'd want a case on this, but, on the other, I wouldn't want to make an already large phone any more bulky than I have to. Either way, a case is not an option here. And speaking of bulk, I really do like how this fits in my hand. Reminds me of how the G3/G4 felt with the Hyperion (I don't have the Hyperion on the V20 to compare) – bulky, but still comfortable because the round back fits natural in hand.
NFC is definitely a perk for anyone who regularly uses Android Pay. If you want or need an extended battery and you also want or need NFC, this is the only option. I have not been able to test the NFC since I don't currently have the battery, but I don't imagine there will be any issues.
It's pretty expensive at $80, but that's only $20 more than the ZeroLemon and that gets you NFC plus this one is so much more comfortable to hold and use. If you haven't bought an extended battery yet and you want a beast of a battery, I'd recommend spending the extra $20 and going Mugen here over ZeroLemon. Even if you have already purchased the ZeroLemon, picking this up wouldn't be such a terrible idea.
If NFC isn't important, though, and you don't need the biggest battery, then the Hyperion is the obvious choice at $30. Even if you need/want the biggest battery, but don't need NFC, you could always buy two Hyperions and swap, though that negates the benefit of never having to swap batteries (depending on your usage).
I'm on the fence about whether or not I want to just go ahead and get this battery or save some money and opt for the Hyperion (or two) with a case. With my usage and signal, I need to swap the stock battery at least twice to get me through a full day so a single Hyperion probably won't cut it. When I was using my ZeroLemon, I could easily get through a full day and then some. Plus I just started using Android Pay...
Hmm I think my mind is made up, just wish I could get a very slim case for this!