I just searched under "cleaner questions" and there were a lot of threads on this so probably better for you to do that!
I agree with IwishIwas I don't think you really know what they will be like till you employ one. Each time I have to get a new one I go through a couple before finding one that suits me.
I currently use an agency but get the same cleaner each week. If you go down the agency route which is handy for holiday cover etc then make sure you will get the same cleaner each week as you don't want to have to keep repeating yourself about how you like things done.