I have setup both Google Apps & BPOS (Microsoft Hosted Exchange)
They are both really simple to setup and offer 25gb mailboxes which is incredible.But I have to say my preference would be with Microsoft's Hosted exchange for the amount of features you get with it.
Like OWA, Outlook Anywhere, Seamless Mobile Integration, Sharepoint Integration.
Although I would go with Google Apps for any site with 5 users or less as Microsoft BPOS requires a minimum of 5 users.
When I first joined CDSOFT I was a big fan of Google Apps & Google's hosted enterprise spam solution - Postini. But we are a dedicated Microsoft Gold Partner and will continue to push Microsoft Solutions.