At Deloitte the qualification you have to do is just the CIMA Certificate in Business Accounting (the foundation qualification for CIMA). You may be exempt from much of this anyway, depending on your degree - check cimaglobal.com in case.
Even if you don't have a business-relevant degree, I wouldn't worry - all the study is frontloaded so you do 4-5 weeks intensive study (with BPP I think) and then take the exam.
I joined direct to the second year of the Analyst programme and already had the CIMA CBA, but in answer to your questions:
a) Yes, it is useful especially if you haven't studied a business degree. It's a good grounding of essential knowledge. It's disturbing that there are some "senior" consultants who wouldn't be able to pass this qualification though.
b) It's not a nightmare by all accounts - possibly a few weeks intense work and you're with other people in the same position so it's a good bonding experience.