This looks really nice. Can't wait to try it out later today. Anyone with a gmail account can use it, and there is no need to install the app if you don't want to.
As a result when using it to message gmail friends their inbox is cluttered with a new thread for every single message I send. I haven't checked if it's due to the subject changing or if they don't properly set In-Reply-To/References, but its mildly annoying for my contacts.
Of course if they are properly setting In-Reply-To and References headers in the message, well then my complaint is really with gmail.