I have the Tommee Tippee warmer, bought it many years back and it is a good help to getting them to the right temp however you still have to watch them and test as if you leave a bottle in too long it can get too hot still.
They work by putting a small amount of water into the top where the bottle goes, when you turn it on it heats up the water which when you put the bottle in warms the bottle evenly, it still takes a couple minutes to come up to temperature and will continue heating if left in.
They are ok, a small jug (just big enough for a bottle to fit it) full of hot (not boiling) water will do exactly the same, the bottle warmer may be ever so slightly faster since it only uses a very small amount of water. They are ok and not that expensive but will be something else you may need to hide when you have company.
These days if I want a bottle of warm milk I have a fully automatic espresso machine with a built in carafe that steams the milk, it has settings for how frothy you want the milk and the lowest is basically no froth but still warmed, it heats up almost instantly so I tend to use that to fill a bottle, perfect temp and no waiting.
