I've been with these guys for almost 5 months already and very happy with them.
I usually book my shifts a week in advance. Staff members try to find suitable shifts for me always, keeping in mind my location and available time. I often have opportunity to choose locations i really like - whenever they have these shifts themselves.
I work only with nurseries in Oxford and surroundings, so I'll tell only about them.
It's very easy job, you just play with the children all day, walk outside. Somites some minor cleaning might be asked - to clean tables after lunch in your room only or to clean up one room in the end of the day. It's 5% of all your working day only, cleaning is usually equally shared between all staff, you will not feel bad about that :)
Most nurseries are very friendly and gave me great feed back and even offrered references. Some are bit colder, you never feel part of the team there. But if it's not really important for you, it will not be a big deal. What I like - you just go there, do your work and go home - no stress, no staff meetings, no staff problems - you know what I mean J
Agency staff seem always happy to hear me though i call pretty often trying to book shifts in advance )
The most enjoyable part of the job - I travelled all Oxfordshire, visited numerous nurseries, saw many people and children. Every day will be different, you will be in different rooms. You are independent. Few times I had to cancel shifts last moment and the staff were absolutety fine and understanding.
The cons - sometimes you get just few days a week. Sometimes - more...