There are plenty of cross-platform messaging apps available, however there are some that are still limited to a single mobile operating system. In this case, one of the apps that is still limited is iMessage, which is available on iOS devices. Or more to the point for today, has previously only been available for iOS devices. A new iMessage app has recently landed in the Play Store -- but there is a catch.
To start, the iMessage app is not from Apple. This makes it an unofficial release, but there is still a bit more to the story. The warning comes by way of Jay Freeman (saurik) who has said that all of the message data is being handled through a server in China. Freeman has said that as far as he can tell, "the way it works is that the client does directly connect to Apple, but the data is all processed on the developer's server in China."
This means that someone using this app will have their message sent from their device to a third-party server in China (which makes it appear to have come from an Apple device) and then sends the message. Basically, those using the app should consider themselves warned about how the app is actually handling these messages. And to further that, you will be required to have an Apple ID, which means giving up a username and password, but remember -- those accounts often have other more personal details attached.
The other aspect here is that Apple may get wise to this and figure out a way to shut them down. Needless to say, the iMessage app for Android is free so you would not be losing out on the app purchase. As for the app itself, the developer has said the primary purpose is to allow everyone to be able to chat using iMessage for free. Otherwise, the app allows the user to send and receive messages and offers goodies such as a read and typing status notifications.
Anyway, those warnings and findings aside, those who want to move forward and begin chatting with their iOS using friends can use this Google Play Store link to download the app.
Update: Google has pulled the iMessage app from Play Store