Download and installation are simple, and each addition of social media requires only a couple of clicks and your login and password information. It makes sense to add as many accounts as you have all at once, because the convenience of Yoono is using a single app for everything.
Of course, no program is perfect, nor is every program perfect for everyone. One drawback of Yoono compared to competitor is that it only shows one pane at a time in a tabbed interface. Sure, it takes up less real estate, but when you want to quickly scan updates, replies and private messages all at once, you can't do that without a bunch of clicking. Plus, the combined feed dispenses with any replies or private messages altogether and only shows status updates. In order to see anything directed at you personally, you have to first click through to the appropriate social network tab, then the replies or private messages tab. It's also worth pointing out that Yoono supports replies and direct messages only for Twitter; Yoono's Facebook integration has only updates, a mostly-useless list of your friends, and a tab for notifications. A tab for your Facebook inbox would be more than welcome.
Another weird design choice is the inability to mess with the options without right-clicking the app in the taskbar or right-clicking somewhere on the screen. Since you'll be tweaking options quite a bit, especially if you add a lot of content to integrate, it would make more sense to have some sort of options icon or obvious dropdown. Also, you can only add a new social network from the taskbar, not the app. Finally, Yoono doesn't auto-update its own feeds, but rather requires you to click whenever it notices more status updates available.
Like Seesmic, Yoono includes a browser-integrated version. Unlike Seesmic, it doesn't just run in the browser, but requires an add-on to your browser and a browser restart to work correctly. Those used to installing their own browser add-ons will have no problem with that.
Despite these quirks, Yoono is convenient just because it can aggregate so much into one app. It's most useful if you have a lot of breadth in social media, but not a lot of depth, Yoono doesn't support many of the deeper uses of social media, such as applications. But Yoono support for MySpace and a slew of instant messaging clients--not just the ubiquitous Facebook and Twitter--gives it an edge over its competitors.