I have the first iPad (not iPad 2) and is updated with ios 5. How do I send messages to other iOS 5 devices? Do I use an email address? Cell phone? Apple iD? Other? The email has worked with a couple of people with iOS 5 (one iPad and one iPhone), but has not with others that I know have iOS 5. What's the deal? Thanks for any help.

I have iPad 1 also . For iMessage I use the Apple ID registered for iCloud.
On the iPhone , there is the same ID and the phone number.
If you want to see your messages on both devices be sure to use the email address (Apple ID) on your phone .

Remember that the messages send via IMessage will be in Blue and regular SMS is in Green.

I've used cell phone numbers, I haven't tried to use email addresses.

I've used iphone numbers ...I can txt people with ios5 that have an iphone

Go to settings > messages

Check to see if everything is set up, if yes, then maybe they didnt set it up yet, check with them since they may not know they need to sign on the first time they use it