Sam is wrong in his understanding on all this, and opinion is off too. Exchange and like are used by the majority of large business and things like this need to work for it properly. Sam is correct in regard to the power of using google, Google apps for a business is great and with things like wave coming and the whole project management, time tracking and even greater google calendar support - Very powerful for business.
Beaker is right about what he is saying, but also does not have the right respect for the online solution. Beaker you do know Google has had less down time then the average internal system in these areas? If there is downtime it effects more people of course but it actually is less likely and you have the updates, upgrades, maintainability, cost (lack of) your end which is great.
A Well designed system doesnt HAVE downtime, the failovers take care of that. Proper Big Business Setup is your primary clusters all tied in at your main datacentre. Your failovers are then bolted to a differnet line, in a different location (Preferably on a different Telephone exchange, with a different power feed, and ideally in a different town). Toed in to that is a decent IT team that doesnt just sit there on their fat arses and do as little as possible, while not caring if things get updated. Plus, you dont even have downtime on the updates because you update one cluster and bring it online before you update the failover (While using the failover to take care of the primarys downtime). That said, the largest company I ever worked for had ZERO failover in any form, and had a piss-poor backup system the likes of which the guy at my corner shop would be horrified if someone suggested he implement it.
Individuals and Small businesses may benefit from SAAS and Cloud Computing, Enterprise Sized companies are just building up a world of hurt if they lose direct control of their IT Assets.