I think BO put ease of use as a very high priority. It's really a user's choice. If you are going to have terabytes of data, complexed schema and perhaps many projects running on these data, choose MicroStrategy. Otherwise, choose BO, or even Cognos. Brio is kinda like Access on Web, an easy going tool also. I like them all.
The average warehouse size for BO customer is around 10 GB. Cognos is under 5 GB. For MicroStrategy, 200 GB. Fit yourself into the right category is pretty easy.