I signed up to use Cloudup a few weeks ago after being impressed by their website's design. Once I received my invitation (the service is still being rolled out gradually) I downloaded the OS X app and started using it in place of CloudApp to see how it compared.
The menu bar app is slick and simple. Dropping a screenshot, video, audio file or almost anything else on the icon plays a chime and immediately starts uploading. It also copies the link to the resource to the clipboard, and opens a browser window to link URL. I disabled the sound and browser window after a few uploads, finding them unnecessary.
Uploads are displayed in a personal stream. I can see the history of items I've uploaded, similar to a Facebook timeline or a Pinterest board. While this is interesting, I question its usefulness in the long term. Having the list of recent items in the menu bar app would probably be enough.
The major feature of Cloudup is streaming uploads: you can share the link to your files before they are finished uploading. The viewer will see the upload progress as well as whatever part of the file has uploaded so far. While it is nice to have the link copied to the clipboard immediately, the only use I found in viewing files before the upload is complete is to see a low-resolution preview of an image.
Days after I started using Cloudup, a CloudApp update was released that contained many new features, such as streaming uploads. But I have yet to try it — for now, my team and I are greatly enjoying the Cloudup experience.