Vantaggi
Despite its size, Bertelsmann is quite an entrepreneurial company. You can be given a lot of liberty to try things out or seek out new clients. There is a big difference between working for the central Bertelsmann company or individual units, which have a very independentist view and have their own culture, in which case you need to look up reviews of those individual units.
Svantaggi
The company remains very German-centric, especially at higher levels, where corporate politics unfortunately take too much space. Your career will be shaped by what they have a need for, not necessarily what you are good at or want to do. It is also interesting to notice how some of their high-fliers leave the firm, only to come back a few years later when the company finally realizes what they have lost.