Yahoo Messenger has been one of the leading instant messaging apps for years. Version 9.0 introduced a whole host of new features, notably in support for video and image links.
One of the more interesting additions is the video support: When you paste in a video link from YouTube and similar video sites, the link is pasted in--and then the video instantly appears inside the chat. This is a nice touch, though the inline video is pretty tiny and can't be resized. This feature also works with links to photos. Not having to click on a favorite link in order to "see" it in chat is a good thing.
One feature introduced in the last few years is the ability to IM back and forth with users of a competing service, Windows Live Messenger. This ability, while useful, would be even better if it also allowed cross-chat with the very popular AOL Instant Messenger, a.k.a. AIM.
Instant messaging is still as popular as ever, though many users have migrated to the more business-oriented Skype service, or simply chat in their webmail. In fact, Yahoo Mail has chat right inside its current Web-based mail service; instead of listing your Yahoo Messenger contacts, it lists everyone in your e-mail address book who's online, a potentially far more useful contact group.
Yahoo Messenger 9.0 also has robust voice support, called Yahoo Voice. Like most other voice over IP solutions, PC to PC calls are free, and the rates for computer to landline or mobile phone are similar to those for other apps like Skype. This requires buying credits in Yahoo Voice; these credits may potentially be less useful overall than credits on Skype.
With Skype leading the way in voice over Internet combined with chat and users of Gmail simply chatting from within that environment--not to mention the megapopularity of text messaging in the last five years--it's possible that standalone instant messenger apps may fall by the wayside, especially if, like this one, the user needs a Yahoo account to use it.
Yahoo has joined the annoying chorus of installers that attempt to do bad things to your browser during the installation process. Be sure and uncheck "Make www.yahoo.com my browser home page" and especially "Make Yahoo my browser's default search engine and enable Yahoo Search Protection to alert me of any attempts to change it." You're probably using Google, with good reason, and have no need to change your search engine.
It might also be a good idea to choose Custom Install and avoid the Yahoo Toolbar with Messenger. At the cost of some screen real estate in the form of an extra bar at the top of your browser, you'll gain access to friend online notifications and one-click links to Yahoo sites. Unless you really despise having to click over to Yahoo Messenger to see who's online, it's probably not necessary.
For those who have both Yahoo Messenger and lots of Yahoo friends (or Windows Live friends), Yahoo Messenger is still essential. Everyone else may opt for text messaging instead--after all the recipient of your text message doesn't have to have an account on your service to get your message. (It's worth mentioning that Yahoo Messenger can send SMS texts, but the recipient can't respond.)
Note: At defaults, this program installs other programs along with it. If you don't want them, be sure to uncheck the boxes during installation.