I moved my email over to a hostage exchange account a couple of months ago. And it's bloomin fantastic!!
All my email available on my desktop and laptop though Outlook or anywhere else through webmail. Also got my Windows Mobile synced to it, so changes in my address book and calendar automatically appear on that.
It's brilliant!!
Got it through Simply Mail Solutions. They are supposed to be moving to Exchange 2007 soon, which will vastly improve OWA.
The only downside is having Microsoft Direct Push turned on kills my phone battery if I don't look after it. Small price to pay though, especially now I'm used to having to charge my phone at night.