Mine is a 3.5 litre one from Tesco - it was £10. I bought it a good few weeks ago, but when I went into the big Tesco in Reading on Saturday they still had quite a few left.
It has a really good heavyweight ceramic casserole inside a stainlesssteel casing. There are three settings - High, Low, and Keep Warm (and Off, of course!!).
I mainly cook casserole type dishes in it - basically just throw in the vegetables and meat, add a bit of liquid, set it on high for half an hour or so, then turn it down to low. You don't need much liquid as the vegetables create juices of their own. However, if you are going to put in something which soaks up liquid, like barley or red lentils, then you will need more.
I think it is best to experiment with some recipes while you are at home to keep an eye on things - when you know what the timings are on your cooker for a particular recipe, then you can go out and leave it.