[One (M9) Accessories] iElvis:
I received one of these from the manufacturer in exchange for a fair review.

I'm a battery junkie because I like to be prepared. I absolutely hate running out of power. As a result, I have a lot of external batteries ranging from a small one that does double-duty as a compact case that I got for my wife to a huge water- and weather-proof one I take camping and boating. The electrons they supply to your phone are all the same; what sets batteries apart are the workmanship and design. I care about design and workmanship, which is why I've owned HTC phones for years.

The Mugen Power 3700mAh Extended Battery Case is is on the small side for external batteries, but that's IMO what you want for a battery case, which can't be so bulky that it interferes with using your phone. You use battery cases either to give yourself extended day-to-day battery life because you're a power user, or for quick top-ups. For larger power needs away from an outlet, you go with a big external battery.

The exterior has a matte finish that feels good in the hand, though it does pick up fingerprints (as you can see in a couple of shots below). The design is sleek and pleasing to the eye. Even my wife, who doesn't care about phones, remarked that it was attractive.

Operation is very simple—you just slide it in and press the power button on the back for a few seconds. There is an LED display on the back, next to the button, that shows the remaining charge.

Fit and finish are excellent. It holds the phone snugly without being too tight.

As with any battery case, it does noticeably add to the bulk of the phone, though not much more than a large wallet case might. It was lighter than I expected.

Charging is via a microUSB port at the base. It charges whether or not the phone is inside, but will it charge the phone first if it's plugged in.

So, it looks good and feels good. How does it work?

It seems to charge at typical 2.1A rates, which means pretty quickly.

Using it, I got a bit more than an extra day out of a full charge on my phone, which is consistent with the stated capacity of the battery given my usual usage. I put it on around 3 p.m. with a nearly full charge, and it was starting to run down the following evening, when my phone was still holding around a 95% charge. Recharging it to full took about 3 hours on a standard power block (I did not run it down to completely empty, which is never a good idea with a li-ion battery).

I did notice some quirks that are probably attributable to HTC's power management. Several times, I noticed that it stopped charging when the phone went to sleep, then would start charging again when it woke up. A couple of times, the phone seemed to get confused by the change, and I had to briefly remove it from the case to restart consistent charging. It also stopped charging twice when it got overheated. Given the charging gremlins this phone has had in the past, I am sure this is an HTC issue rather than a MugenPower issue. In any case, it wasn't a big deal.

My sole concern is that, as you can see in the photos above, front protection is limited. The lip of the case extends only a very short distance above the screen. If you have a tempered glass screen protector (I don't), it's possible it might stick out above it or at least be nearly flush with the case. But beyond that, I found nothing to stop me from recommending this if you're looking for a good battery case.

Update: After using this for a week and a half, I really like it. I've gotten used to the bulk, though I don't use it all the time. The phone does seem to get confused by the charging, though. It will charge fine to 100%, but if you keep using it, it shifts to a continual trickle charge that never quite gets to 100% unless you take the case off for a few seconds and put it back. This is HTC's problem, though, as I've seen people with the same issue on wall chargers.