Push e-mail, I'm assiming using the MS exchange push e-mail option on the Vario II:
Advantages:
Instant e-mail (duh!)
Geeky/coolness factor
Can be a life saver if you you live/die by e-mail
Disadvantages:
Increased battery drain (quite an increase)
higher data charges
Can cause WM5 devices to freeze more often (blame activesync!)
Take your pick.
If you get a lot of e-mails, or need to carry a lot of e-mails with you (like I do) you can tweak the registry to store e-mails on the memory card. Which is pretty handy when you need access to 300mb worth of e-mails....