Having tried out Yahoo! Messenger, I have to say that it, when compared to some of the more pleasant systems such as MSN and AIM, is rather ghastly to use.
By default, it has a highly annoying "IMVironment" system, not unlike the skinning systems commonly featured in some of the other Instant Messaging systems. But Yahoo! takes it one step further - any "IMVironment" you load into a conversation window will also get loaded by the person you're talking to.
One of these IMVironments makes most of the conversation window get taken up by a large sketch pad, of which you can draw in, and anything you draw will be synchronised with your contact.
Also, in the newer editions of Yahoo! Messenger. there is an absolutely satanic option in one of the menus at the top of each conversation window entitled "Buzz Friend". This option will effectively play a doorbell sound on the machine of whoever you are talking to, 'shake' the message window around (not unlike an earthquake), and display, in capital bold red letters, the word "BUZZ!!!" in the message window.
Why Yahoo! chose to include this highly annoying and stupid feature I have no idea. (Note: It seems there's a time delay between how often you can 'BUZZ!' people, 'BUZZ!'ing people can also cause things to happen in the loaded IMVironment.)
It also seems that the chatroom function fails silently if the account you have signed in with has a registered birthdate of 13 or lower. Also, a horrendously large banner ad takes up a considerable amount of space when using the chatrooms, which is forcibly tacked on to the bottom of the chatroom window.