I recently tried out the Wahl Bling/G-whiz trimmer a great looking cordless compact trimmer that runs on a single AA battery. While small, the Bling/G-whiz is quite powerful and the three guards (included) are steady and don't pop out.
It is great for trimming beards, mustaches, other facial hair and quick line ups. It's great for traveling with, just throw it in your bag and be on your way.
Initially, the trimmer performs very well for a few months without complaint. Then it seems to work sporadically, even after changing batteries, before totally burning out.
But at $19- $24.00 USD, these little trimmers can be replaced every few months if that's what you want to do.