The Monoprice battery has worked very well since I purchased it but I wanted to have a battery with a greater capacity (and it doesn’t hurt to have an additional battery as well). I always have my iPhone with me and I often carry a mifi and the 3DS as well. In addition, sometimes I’m traveling with others that have a device that could use a recharge.
Last March I purchased an Anker Astro3E 10000mAh External Battery.
Functionality of the device is similar to the Monoprice battery so I won’t bother with describing the minor details. Overall, it’s similar but clearly does provide greater battery capacity. It’s been used several times and I haven’t had a problem with the device itself.
I have noticed an odd issue with my portable Powermat charging mat – when the battery is fully charged (or nearly fully charged) the mat will turn the charging on and off repeatedly, which can be very annoying if the charging sound on the mat is enabled and it happens in the middle of the night. However, this isn’t an issue with the battery and I’ve experienced a few oddities with the portable charging mat and other devices.
Updated 05/22/2013: I’ve been using this battery for a while and so far it has worked great. I’ve had the opportunity to use it on several trips and I’m actually using this battery more than I am the Monoprice battery.
Updated 07/06/2014: This battery still works great and is extremely useful when traveling.