See attached document prepared by AT KEARNEY. It is one of the well prepared documents about BI I have seen recently. AT Kearney has definetely good approach from top. Nevertheless, they underlined the complexity of BI as you can see in the picture. The most important fact here is that how to define roadmap, where most companies fail. Some companies choose to go with big vendors since they have been working with them for years while the others choose “best of breed approach”.
I am a BI person who prefers “best of breed approach” because none of these big guys has strong integration, because they did not re-write the tools they acquired. In current technology word, you can integrate anything in data level though. Anyhow, I like the AT Kearney document and advise people to read instead of searching many sites..it is a good one.