arguably the worst part about imessage and scrolling is that even if you do manage to scroll up and find a previous message, you are automatically scrolled down to the bottom if a new message is received in that chat (on iphone)
yes it is really bad, the UIs for PMs was better 10 years ago, thinking about ICQ, Adium, MSN Messenger and the like. You could export history as file and easily scroll and search them