Moving your Exchange services to Office 365 seems to be a fairly simple decision for smaller companies; it just makes sense. Most of my larger customers however remain on-prem due to security concerns and higher estimated costs. In addition, there are often political challenges in eliminating local servers and eliminating jobs within IT departments and sometimes irrational fears of a global cloud shutdown or problem. (Blackberry/RIM problems come to mind.)
The slower adoption rate of Office 365 for larger entities is a huge topic and I would need some help in covering that completely, but I did find an interesting article today with price comparisons for small shops. It’s the first time I have seen calculations and a nice chart that shows the costs between the two options for various small company sizes. This article suggests that the price advantage with Office 365 starts to wither once you reach 1,000 mailboxes.
Anyway, check it out.