Advantages:
Able to text on wifi
Able to use email user name instead of actual phone number and to use it as a caller ID when sending. Also the ability to change it anytime.
International texts/mms for free and no need to worry about any long distance or roaming charges.
Can be used on any iOS5 device.
Have iMessages received on all iOS5 devices at once. Note: Does not work with phone number. Must use Apple ID and/or Manually add email user names to all devices.
More instant/faster than regular text.
Delivered, Read Receipts and feedback of when someone is currently typing.
Completely integrated & easy to disable. No app to open.
Tens of millions own a iOS5 device.
A bit more privacy, iMessage does not reflect on your bill. Regular texts do reflect in bill who text and when.
Able to send higher quality and longer Vids.
Disadvantages:
No cross platform and most likely never will be.
No OSX or Windows use. (Might be implemented at least on OSX in the near future)