The Messages App can do everything iChat can and has the ability to iMessage to iOS devices as well.
iChat and the Messages app need to both be using either AIM or Jabber based IDs (AIM to AIM, Jabber to Jabber)
If you have iChat 6 and they have Messages you could also both use Yahoo IDs (Text Only)
Google via GoogleTalk have as Jabber Server.
Facebook enables their IDs to be used on a Jabber server they run.
There are many more Jabber servers out there.
AIM valid names can include Apple ID sending in @mac.com, @me.com and @icloud.com
The last two are likely to have been issued via iCloud and will only work in iChat 6 and Messages.
(It's about the way Apple now gets AIM valid IDs passwords checked in those two apps)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously