I just got a new Dell X51 with Mobile 5.0 on it. Overall not much about the OS is really any different. Slightly different UI style but everything is essentially the same from a user perspective.
Activesync 4.0 is a little different. The PDA actually has a “Local Area Connection” entry in the computer's network settings. So maybe it's Internet connectivity will be more reliable now. It also didn't ask a bunch of stupid questions when I went to install our software on to it.