I'm the SBM of a secondary academy. The best bits are the sheer variety of my role - never moment to be bored and always something else to think about. i have never worked so hard, but love my job. There are tough days, and sometimes it's hard being the bad guy all the time (being a SBM will not make you popular if you are making tough decisions about expenditure), but I wouldn't give up my job for anyone. The challenges are balancing all the competing priorities, trying to make time to be strategic (rather than getting bogged down in the operational), and getting teachers to comply with financial procedures (often like herding cats).
I second what has been said above - in my school, versatility, the ability to handle competing demands under pressure, and a willingness to do what it takes to get the job done is absolutely paramount (including, in my case, brandishing cleaning cloths alongside my cleaning team to get new classrooms ready the day before term started). In my school we do whatever it takes for the kids - after all, that is why we are there.
Finance is only a part of what I do - the role can encompass everything 80smum mentions, but how far in each area depends on the school (and, more particularly, the headteacher). You need to know where the strengths are already - so, for example, when I applied for my job I knew there was a strong HR officer for the school, so didn't need to show indepth knowledge of HR, but H&S management was a weakness, so I used that to my advantage. Use the visit to scope these things out if you don't know the school already.
One of the most important skills is knowing a little about a lot - being able to assimilate information quickly and become a temporary expert (or at least talk knowledgeably) with professionals from different areas of expertise (accountants, lawyers, gas engineers, unions etc). Did I know everything there was to know about asbestos management/solenoid valves/accruals/Burgundy book before I started the job? Nope. Did I rapidly learn more than I ever thought I wanted to know about these things, to the point of being able to talk to the relevant people about them? Hell yes.
In some organisations (academy chains particularly) being an accountant is a way of filtering candidates for the SBM and - increasingly - FD roles, but it isn't the be-all and end-all. It really depends on the type of school - mine is quite forward thinking and was open to someone without an accountancy background in the role because their finance function was reasonably strong anyway, so other things became more important.
Oh, and don't take the job because you want school holidays off! This summer will be the first time since I started we haven't had major building work taking place...