Manchester is a great program, it's ranked right up there with Oxford and Cambridge.
Having attended both Manchester at undergrad and a top US school on exchange, I can tell you that it will be academically tougher than the US schools purely because of the way University is in the UK, however the networking wont be as good. I've lived in Manchester before and the city is great so in terms of work life balance it's a good place to be. The school definitely adds prospects, I know that the current class president just accepted a six figure sum, as with all b-school you get out what you put in.