We are in the process of doing something similar, initially we were very reassured, lots of talk about independence, retaining our own individuality etc, but as we have gone further into the process, things are beginning to look slightly different! The costs are based on the OFSTED grading of the school, and are fixed. The pro's? Well, more centralised support ( rather than the mish-mash of training schools/teaching alliances/cluster schools we seem to have at the moment) centralised admin services ( and the ability to move away from the current LA provider which has been cut to the bone) a MAT which in theory has the same ethos as the school. The con's? Rather more control than initially thought, we have to use the MAT progress tracker and assessment schedule, the curriculum is "under discussion," the MAT SIP has very definite views on paperwork and policies!
We are still going in ( we don't have much option!)and the MAT is possibly one of the better ones in the area but we are still cautious!