IMHO, I find it useless. Maybe for long messages (splitted, >500chars) in East-European languages it's a good idea, but not for any Western European language (English, French, Spanish... yeah, it's maybe a little bit slower to write but not a pain, at all).

Anyway, the whole process (open the app, write in my computer then copypaste to the message app) is slow and painful. It would be "better" to have an associated username (with password) in the computer+in the iPhone. For example: I open the app, I write in my computer that long-long message, the app automatically updates (COMET anyone?) and I just tap a "send" button, which opens the message app with the message already written...

... but in fact, the whole idea is pretty useless, we don't write messages as we write emails. And for emails, we already have another way to do it :-)