We use Rackspace's Hosted Exchange service at my office. I believe it is about $10/user/month.
It works well and has all the features that you'd expect from Exchange but you don't manage anything. We have Outlook 2003, 2007, and 2010 clients. I have also configured iPods via IMAP/SMTP, so I believe it would work just fine with Outlook Express.
BlackBerry Enterprise Server can be enabled on an individual user basis for an additional $10/phone/month. Calendar/Contact sharing works properly.
ActiveSync for iPhones, Android, and Windows mobile is $3/phone/month.
There is also a "Hybrid Exchange" setup where not all of your accounts need to be actual Exchange users. We do not use this, but it looks interesting.
Data Point: We have 20 users on this system with 1 BlackBerry. Everyone is happy, especially me, since now I don't have to manage an Exchange Server :)