If you're sending to a GSM phone, the quickest way to send to someone is to select their number from AddressBook and tap the SMS button.  Type in the message and press Send.  Very difficult .

SMS isn't SMTP, and messages may take time to receive.  I sent myself an SMS message using VoiceStream's network and it took twenty minutes to get it.  The rest of my test messages took less than a minute.

The SMS Application doesn't have an email lookup function (only phone numbers), so if you are in the application you will have to manually type in someone's email address.  After selecting "SMS E-Mail Center" in the To field, begin your message text with the recipient's email address followed by a space and a # symbol (i.e. [email protected] #).  Then type in your message like normal.

The SMS Application is also what notifies you when you have a new voice mail message from someone.  One nice thing is that if you are using VoiceStream, you will receive an SMS message with the person's name and phone number if available that lets you know you have a voice mail and gives you the option to immediately call them back.  The message is displayed almost immediately after inserting the VisorPhone module, which is important since the VisorPhone only works when inside the Visor.

Other cellular providers may provide voice mail notification in a different manner.  I believe that PacBell displays a small icon when you have a voice mail instead of sending you an SMS message.