Even for $7, I say that they are a waste of space and money.
I was gifted one and it took FOREVER to even remotely come close to warming a bottle. Also, bottles are not required to be warmed before giving to an infant, so I would first see if your LO will take a cold to room temperature bottle before even bothering to find what warming method works best for you.
ETA: (I just saw your ETA) I don't know how long your work day is, but expressed breast milk can sit at room temperature for up to six hours, and be A-okay. When I was an exclusive pumper with DS, if I was going somewhere and pumping, I'd pump the milk into bottles and then screw on the cap that doesn't have a nipple attached. That way, I wouldn't have to worry about heating up the bottle if I knew that I was going to be feeding DS within that 6 hour period. (Room temperature was warm enough to his liking)