Does iMessage cost money?

If the other party is also an iPhone user and uses mobile data or wireless network when sending text messages, then iMessage is free.

iMessage can tell you whether the message has been delivered. If the recipient has enabled read receipts, you can also see the receipt message that the message has been read. iMessage can also display status information when someone is replying, allowing you to continue the conversation naturally and smoothly. When the text bubble is blue instead of green, you know you're using iMessage instead of SMS and will receive a reply faster.