A bit of an expensive way to heat your wine! Very cosy though!
There's two main types. Thermostatic and not thermostatic.
Thermostatic means the heater switches itself off when the wine gets warm enough.
The thermostatic ones are pre-set to 24 C but are adjustable. They are a higher energy rating but if you lag the fermenter they actually work out more economic. Especially if you plan to use it in a garage or somewhere with no heat.
They are made of glass, so they're fragile and you need to sterilise them because they sit in the wine/beer
Some people don't like the thought of having electrical items in their wine/beer!
Non Thermostatic means they just get warm and stay switched on. A bit like a light bulb. If the weather gets hot they can overheat the wine, if it's cold the container needs to be lagged a bit, they are simple to use and since they go outside the fermenter don't need sterilising.
Both have their faults - Both have their good points!
A stick on thermometer is a quick easy way to check the temperature of your wine/beer!
The back of the fridge gives off a lot of heat, you may be able to stand your wine/beer on top of the fridge- this works very well for some people! I wouldn't try it with one of those tall larder fridges though...