The launch of this service hasn’t been Apple’s finest hour and I would caution anyone who is thinking of either trialling or signing up to it to make a reliable backup of everything before taking the plunge. Try reading David Pogue’s article in the New York Times and then reconsider if you want to take the risk.
My experience (laptop, iMac, no iPhone) is that it mostly works except for a few oddities with calendars in the web interface. However, there is a problem on my MacBook in that every time it syncs with the server, it hangs for about 15 seconds – as in everything locks out or stops (Mail, Safari, iTunes). So I’ve switched off that option and only sync manually. Not good. I do not have the same problem with the iMac at home.