Well, can it successfully send and/or receive messages sent through the system?
I've used version 5.6, which is currently the stable version. This is the version Yahoo has out for download to convince the internet public that they can program a digital Hermes. To be successful, all the program would have to do is have your message arrive on my screen without dire consequence. As of 5.6, Yahoo can't manage that simple task. It fails, and it fails spectacularly!
This is what happens. Someone notices me online by the little lit up icon and decides to send me a greeting, perhaps to cheer me up. They type it up and press "Enter". It travels through the internet, finding its way to my computer. Y!IM, listening over the internet for any messages addressed to me will gladly accept it. Now at this point, you want your messaging program to create a window on your screen so it can relay this message that was sent. Now, I can not begrudge Yahoo for not trying, for yes it tries. Oh, it tries. It tries very hard. It tries and tries and tries and tries, all the while monopolizing the resources of my machine. Yahoo will easily grab 99% of the CPU, while remaining unsuccessful in creating the window to deliver the message.
Now, if it's taking up 99% of my computer's time and energy to fail in making a window, imagine if I had been doing anything else with this machine besides listening for Y!IM users trying to contact me. I'd see my 1.8GHz computer mysterious slow to molasses without any visible reason. Or if I had just decided to start work, for Yahoo has frequently done this when it was the only program running, I'd see my desktop as responsive as a painted mural. Yahoo has not only failed in its basic function as a program, which is to deliver messages sent to me, it has failed in the basic operation of a program designed for a graphical user interface — creating a window! Worse, it has failed in a way so that there is no way to know why my computer has frozen up.
I don't know how I discovered this, but one day I right clicked the Y! icon in the system tray. Lo and behold — five windows from Yahoo Messenger appeared, and in each was a single message sent from the the same user, concluding in wondering why I haven't messaged her back. I did not take note of the time my computer started slowing to tar so I had no idea how long these messages had been sitting in there, clogging up the system. All I know is after she came back online, it took me 20 minutes of explaining why I was ignoring her.
Let's see... Avatars and Audibles probably require additional abuse of Flash by Yahoo (IMVironments first introduced this system to that accursed Macromedia ActiveX control.). Launchcast Radio must require integration of another entire program into Y!IM. There must be a contest at Yahoo Labs to see who can go fastest from 60 to zero!
Now, AOL Instant Messenger has the obnoxious habit of delivering advertisements with audio when it is running. Microsoft wants to integrate everything on earth two ways between Windows XP and Passport which as I write this is still one of their .NET tools. Yet, both have mastered its basic function of delivering messages, and without monopolizing my CPU. I'd love to boycott Y!IM, but I can't convince many friends to move with me to AOL or Microsoft. Thus to keep my head from leaving imprints on my keyboard, I keep Trillian running.
But now, I'm back to being begged to install the Yahoo Messenger beta, so we can play with the cute stuff on it. Amazingly, she can't understand why I hold Yahoo's inability to deliver messages as a reason against using it as a messaging program. "But it's cute!" she says. I have no rebuttal to that.